[X86] combineMulToPMADDWD - replace ASHR(X,16) -> LSHR(X,16)

If we're using an ashr to sign-extend the entire upper 16 bits of the i32 element, then we can replace with a lshr. The sign bit will be correctly shifted for PMADDWD's implicit sign-extension and the upper 16 bits are zero so the upper i16 sext-multiply is guaranteed to be zero.

The lshr also has a better chance of folding with shuffles etc.