[x86] scalarize extract element 0 of FP cmp

An extension of D58282 noted in PR39665:
https://bugs.llvm.org/show_bug.cgi?id=39665

This doesn't answer the request to use movmsk, but that's an
independent problem. We need this and probably still need
scalarization of FP selects because we can't do that as a
target-independent transform (although it seems likely that
targets besides x86 should have this transform).

llvm-svn: 355741