JVM IR: simplify chains of negations in if conditions.

Introduce lowering phase that turns !!exp -> exp for the boolean
'not' builtin. This makes sure that code such as

```
if (!!!!!booleanValue) {
    doStuff()
}
```

generates only one branch.

compiler/testData/codegen/bytecodeText/lazyCodegen/negateVarChain.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 val two = 2
 
 fun test2() {