Optimize coercion to Unit.

POP operations are backward-propagated.

Operation can't be transformed if its result is moved within stack
(by DUP, DUP_X1, DUP_X2, DUP2, DUP2_X1, DUP2_X2, or SWAP).

Unnecessary operations are replaced with NOPs (for debugger).
Unnecessary NOPs are removed.

KT-9922 Suboptimal generation for simple safe call with unused return value
KT-11116 Optimize methods returning Unit

compiler/testData/codegen/bytecodeText/coercionToUnitOptimization/nopInlineFuns.kt

@@ -0,0 +1,26 @@
+fun test() {
+    val a = inlineFunInt { 1 }
+    val b = simpleFunInt { 1 }
+    val c = inlineFunVoid { val aa = 1 }
+    val d = simpleFunVoid { val aa = 1 }
+}
+
+inline fun inlineFunInt(f: () -> Int): Int {
+    val a = 1
+    return f()
+}
+
+inline fun inlineFunVoid(f: () -> Unit): Unit {
+    val a = 1
+    return f()
+}
+
+fun simpleFunInt(f: () -> Int): Int {
+    return f()
+}
+
+fun simpleFunVoid(f: () -> Unit): Unit {
+    return f()
+}
+
+// 3 NOP

compiler/testData/codegen/bytecodeText/coercionToUnitOptimization/returnsUnit.kt

@@ -0,0 +1,4 @@
+fun test() = Unit
+
+// 0 GETSTATIC
+// 0 POP

compiler/testData/codegen/bytecodeText/coercionToUnitOptimization/safeCall.kt

@@ -0,0 +1,10 @@
+interface A {
+    fun foo()
+}
+
+fun test(x: A?) {
+    x?.foo()
+}
+
+// 1 POP
+// 0 ACONST_NULL

compiler/testData/codegen/bytecodeText/coercionToUnitOptimization/safeCallWithReturnValue.kt

@@ -0,0 +1,10 @@
+interface A {
+    fun foo(): Any
+}
+
+fun test(x: A?) {
+    x?.foo()
+}
+
+// 2 POP
+// 0 ACONST_NULL

compiler/testData/codegen/bytecodeText/coercionToUnitOptimization/safeLet.kt

@@ -0,0 +1,19 @@
+fun test(ss: List<String?>) {
+    val shortStrings = hashSetOf<String>()
+    val longStrings = hashSetOf<String>()
+    for (s in ss) {
+        s?.let {
+            if (s.length < 4) {
+                shortStrings.add(s)
+            }
+            else {
+                longStrings.add(s)
+            }
+        }
+    }
+}
+
+// 2 POP
+// 0 INVOKESTATIC java/lang/Boolean\.valueOf
+// 0 CHECKCAST java/lang/Boolean
+// 0 ACONST_NULL

compiler/testData/codegen/bytecodeText/coercionToUnitOptimization/tryInlined.kt

@@ -0,0 +1,10 @@
+// WITH_RUNTIME
+
+import java.io.*
+
+fun test(r: Reader) {
+    val ss = hashSetOf<String>()
+    r.useLines { it.forEach { ss.add(it) } }
+}
+
+// 2 POP