Add a new control flow simplifier.

Run it in the dead code elimination phase, as it relates to
creating dead branches.

From 0.04 to 0.07% less code size framework/gms/docs/fb (70K saved on fb)
3%-5% runtime performance improvements on Richards/DeltaBlue/Ritz.
Compile-time is mixed, so in the noise (from 2% slower to 1% faster).

test:611-checker-simplify-if

Change-Id: Ife8b7882d57b5481f5ca9dc163beba655d7e78bf