[X86] Directly emit X86ISD::PMULUDQ from the ReplaceNodeResults handling of...

[X86] Directly emit X86ISD::PMULUDQ from the ReplaceNodeResults handling of v2i8/v2i16/v2i32 multiply.

Previously we emitted a multiply and some masking that was supposed to matched to PMULUDQ, but the masking could sometimes be removed before we got a chance to match it. So instead just emit the PMULUDQ directly.

Remove the DAG combine that was added when the ReplaceNodeResults code was originally added. Add a new DAG combine to avoid regressions in shrink_vmul.ll

Some of the shrink_vmul.ll test cases now pick PMULUDQ instead of PMADDWD/PMULLD, but I think this should be an improvement on most CPUs.

I think all of this can go away if/when we switch to -x86-experimental-vector-widening-legalization

llvm-svn: 350134