[TargetLowering] make helper function for SetCC + and optimizations (NFC)

After looking at D19087 again, it occurred to me that we can do better. If we consolidate
the valueHasExactlyOneBitSet() transforms, we won't incur extra overhead from calling it a
2nd time, and we can shrink SimplifySetCC() a bit. No functional change intended.

Differential Revision: http://reviews.llvm.org/D20050

llvm-svn: 268932