[X86] In combineMOVMSK, look through int->fp bitcasts before callling SimplifyDemandedBits.

MOVMSKPS and MOVMSKPD both take FP types, but likely the operations before it are on integer types with just a int->fp bitcast between them. If the bitcast isn't used by anything else and doesn't change the element width we can look through it to simplify the integer ops.

llvm-svn: 341915