For comparison intrinsics and for instanceof checks, make
it possible to get the the stack value produced and branch
on that directly instead of materializing a boolean to
branch on from it.
That reduces code such as
```
IF_CMPEQ L1
CONST_0
GOTO L2
L1: CONST_1
L2: IFEQ L3
```
to just one IF_CMP instruction.
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.
- Turn some const conditions into non-const conditions
- Make sure inlined const values are used where required
(otherwise they are eliminated by POP backward propagation)
