[InstCombine] try to convert x86 movmsk intrinsic to generic IR (PR39927)

call iM movmsk(sext <N x i1> X) --> zext (bitcast <N x i1> X to iN) to iM

This has the potential to create less-than-8-bit scalar types as shown in 
some of the test diffs, but it looks like the backend knows how to deal 
with that in these patterns. This is the simple part of the fix suggested in:
https://bugs.llvm.org/show_bug.cgi?id=39927

Differential Revision: https://reviews.llvm.org/D55529

llvm-svn: 348862