JVM_IR: Optimize null checks.

Introduce lowering to remove null checks for primitive type
expressions and replace them with true/false. Side-effects
are preserved.

Generate ifnull/ifnonnull instructions for null checks instead
of materializing a null literal for an equality check.

compiler/testData/codegen/bytecodeText/deadCodeElimination/boxing.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 class A
 
 fun foo(x: Any?) {}

compiler/testData/codegen/bytecodeText/deadCodeElimination/literal.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 fun box() {
     val x: Any? = "abc"
     val y: Any? = if (1 == 1) x else "cde"

compiler/testData/codegen/bytecodeText/deadCodeElimination/simpleConstructor.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 class A
 fun box() {
     val x: A? = A()

compiler/testData/codegen/bytecodeText/deadCodeElimination/simpleConstructorNotRedundant.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 class A
 fun box() {
     val x: A? = A()