[X86] Emit native IR for pmuldq/pmuludq builtins.

I believe all the pieces are now in place in the backend to make this work correctly. We can either mask the input to 32 bits for pmuludg or shl/ashr for pmuldq and use a regular mul instruction. The backend should combine this to PMULUDQ/PMULDQ and then SimplifyDemandedBits will remove the and/shifts.

Differential Revision: https://reviews.llvm.org/D45421

llvm-svn: 329605