[PowerPC] eliminate redundant compare instruction

If multiple conditional branches are executed based on the same comparison, we can execute multiple conditional branches based on the result of one comparison on PPC. For example,

if (a == 0) { ... }
else if (a < 0) { ... }

can be executed by one compare and two conditional branches instead of two pairs of a compare and a conditional branch.

This patch identifies a code sequence of the two pairs of a compare and a conditional branch and merge the compares if possible.
To maximize the opportunity, we do canonicalization of code sequence before merging compares.
For the above example, the input for this pass looks like:

cmplwi r3, 0
beq    0, .LBB0_3
cmpwi  r3, -1
bgt    0, .LBB0_4

So, before merging two compares, we canonicalize it as

cmpwi  r3, 0       ; cmplwi and cmpwi yield same result for beq
beq    0, .LBB0_3
cmpwi  r3, 0       ; greather than -1 means greater or equal to 0
bge    0, .LBB0_4

The generated code should be

cmpwi  r3, 0
beq    0, .LBB0_3
bge    0, .LBB0_4

Differential Revision: https://reviews.llvm.org/D37211

llvm-svn: 312514