Fix VerifyError in coroutines caused by null spilling

While within a method by the JVM spec null-value has a special
Nothing-like type, when we spill it for a coroutine, we must choose
some real type to CHECKCAST to after restoring the variable's value.

But the problem is that such a real type depends on usage of that null value,
and there may be more than one usage.

The solution is not to spill such variables into fields, but instead
init them with ACONST_NULL after each suspension point

 #KT-16122 Fixed

compiler/testData/codegen/box/coroutines/varSpilling/nullSpilling.kt

@@ -0,0 +1,45 @@
+// WITH_RUNTIME
+// WITH_COROUTINES
+import kotlin.coroutines.experimental.*
+import kotlin.coroutines.experimental.intrinsics.*
+
+suspend fun foo(value: String): String = suspendCoroutineOrReturn { x ->
+    x.resume(value)
+    COROUTINE_SUSPENDED
+}
+
+fun bar(x: String?, y: String, z: String): String {
+    if (x != null) throw RuntimeException("fail 0")
+    return y + z
+}
+
+suspend fun baz1(): String {
+    return bar(null, foo("O"), foo("K"))
+}
+
+suspend fun baz2(): String {
+    var x = null
+
+    for (i in 1..3) {
+        x = null
+    }
+
+    return bar(x, foo("O"), foo("K"))
+}
+
+fun builder(c: suspend () -> Unit) {
+    c.startCoroutine(EmptyContinuation)
+}
+
+fun box(): String {
+    var result = ""
+
+    builder {
+        result = baz1()
+
+        if (result != "OK") throw RuntimeException("fail 1")
+        result = baz2()
+    }
+
+    return result
+}

compiler/testData/codegen/light-analysis/coroutines/varSpilling/nullSpilling.txt

@@ -0,0 +1,34 @@
+@kotlin.Metadata
+public final class CoroutineUtilKt {
+    public final static @org.jetbrains.annotations.NotNull method handleExceptionContinuation(@org.jetbrains.annotations.NotNull p0: kotlin.jvm.functions.Function1): kotlin.coroutines.experimental.Continuation
+    public final static @org.jetbrains.annotations.NotNull method handleResultContinuation(@org.jetbrains.annotations.NotNull p0: kotlin.jvm.functions.Function1): kotlin.coroutines.experimental.Continuation
+}
+
+@kotlin.Metadata
+public class EmptyContinuation {
+    public final static field Companion: EmptyContinuation.Companion
+    private final @org.jetbrains.annotations.NotNull field context: kotlin.coroutines.experimental.CoroutineContext
+    inner class EmptyContinuation/Companion
+    public @synthetic.kotlin.jvm.GeneratedByJvmOverloads method <init>(): void
+    public method <init>(@org.jetbrains.annotations.NotNull p0: kotlin.coroutines.experimental.CoroutineContext): void
+    public synthetic method <init>(p0: kotlin.coroutines.experimental.CoroutineContext, p1: int, p2: kotlin.jvm.internal.DefaultConstructorMarker): void
+    public @org.jetbrains.annotations.NotNull method getContext(): kotlin.coroutines.experimental.CoroutineContext
+    public method resume(@org.jetbrains.annotations.Nullable p0: java.lang.Object): void
+    public method resumeWithException(@org.jetbrains.annotations.NotNull p0: java.lang.Throwable): void
+}
+
+@kotlin.Metadata
+public final static class EmptyContinuation/Companion {
+    inner class EmptyContinuation/Companion
+    private method <init>(): void
+}
+
+@kotlin.Metadata
+public final class NullSpillingKt {
+    public final static @org.jetbrains.annotations.NotNull method bar(@org.jetbrains.annotations.Nullable p0: java.lang.String, @org.jetbrains.annotations.NotNull p1: java.lang.String, @org.jetbrains.annotations.NotNull p2: java.lang.String): java.lang.String
+    public final static @org.jetbrains.annotations.Nullable method baz1(@org.jetbrains.annotations.NotNull p0: kotlin.coroutines.experimental.Continuation): java.lang.Object
+    public final static @org.jetbrains.annotations.Nullable method baz2(@org.jetbrains.annotations.NotNull p0: kotlin.coroutines.experimental.Continuation): java.lang.Object
+    public final static @org.jetbrains.annotations.NotNull method box(): java.lang.String
+    public final static method builder(@org.jetbrains.annotations.NotNull p0: kotlin.jvm.functions.Function1): void
+    public final static @org.jetbrains.annotations.Nullable method foo(@org.jetbrains.annotations.NotNull p0: java.lang.String, @org.jetbrains.annotations.NotNull p1: kotlin.coroutines.experimental.Continuation): java.lang.Object
+}