[PatternMatch, InstSimplify] allow undef elements when matching any vector FP zero

[PatternMatch, InstSimplify] allow undef elements when matching any vector FP zero

This matcher implementation appears to be slightly more efficient than
the generic constant check that it is replacing because every use was
for matching FP patterns, but the previous code would check int and
pointer type nulls too.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@327627 91177308-0d34-0410-b5e6-96231b3b80d8