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()
```
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.
This proved to be a fragile technique, which probably doesn't even improve
performance in most cases but has lots of unexpected problems: unconditional
initialization of reflection classes, increasing the size of the bytecode, bugs
with <clinit> in annotations on JVM 6, inability to support conversion of a
class from Kotlin to Java without recompiling clients which use it
reflectively, etc.
