JVM_IR: Generate better code for null checks.

Simplify ifs when branches have condition true/false.

Simplify blocks containing only a variable declaration
and a variable get of the same variable. Simplify to
just the condition.

Do not introduce temporary variables for constants for
null checks. Constants have no side-effects and can be
reloaded freely instead of going through a local.

This simplifies code such as "42.toLong()!!" so that the
resulting code has no branches and uses no locals. The
simplifications happen as follows:

```
block
  temp = 42.toLong()
  when
    (temp == null) throw NPE
    (true) load temp

---> null test simplification

block
  temp = 42.toLong()
  when
    (false) throw NPE
    (true) load temp

---> when simplification

block
  temp = 42.toLong()
  load temp

---> block simplification

42.toLong()
```

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/IrUtils.kt

@@ -144,6 +144,9 @@ fun IrMemberAccessExpression.addArguments(args: List<Pair<ParameterDescriptor, I
 
 fun IrExpression.isNullConst() = this is IrConst<*> && this.kind == IrConstKind.Null
 
+fun IrExpression.isTrueConst() = this is IrConst<*> && this.kind == IrConstKind.Boolean && this.value == true
+
+fun IrExpression.isFalseConst() = this is IrConst<*> && this.kind == IrConstKind.Boolean && this.value == false
 
 fun IrExpression.coerceToUnitIfNeeded(valueType: KotlinType, irBuiltIns: IrBuiltIns): IrExpression {
     return if (KotlinTypeChecker.DEFAULT.isSubtypeOf(valueType, irBuiltIns.unitType.toKotlinType()))