[PowerPC] optimize conditional branch on CRSET/CRUNSET

This patch adds a check to optimize conditional branch (BC and BCn) based on a constant set by CRSET or CRUNSET.
Other optimizers, such as block placement, may generate such code and hence
I do this at the very end of the optimization in pre-emit peephole pass.

A conditional branch based on a constant is eliminated or converted into unconditional branch. 
Also CRSET/CRUNSET is eliminated if the condition code register is not used
by instruction other than the branch to be optimized.

Differential Revision: https://reviews.llvm.org/D52345

llvm-svn: 343100