[PatternMatch] allow undef elements when matching vector FP +0.0

This continues the FP constant pattern matching improvements from:
https://reviews.llvm.org/rL327627
https://reviews.llvm.org/rL327339
https://reviews.llvm.org/rL327307

Several integer constant matchers also have this ability. I'm
separating matching of integer/pointer null from FP positive zero
and renaming/commenting to make the functionality clearer.

llvm-svn: 328461