[X86] Teach shouldSinkOperands to recognize pmuldq/pmuludq patterns.

The IR for pmuldq/pmuludq intrinsics uses a sext_inreg/zext_inreg
pattern on the inputs. Ideally we pattern match these away during
isel. It is possible for LICM or other middle end optimizations
to separate the extend from the mul. This prevents SelectionDAG
from removing it or depending on how the extend is lowered, we
may not be able to generate an AssertSExt/AssertZExt in the
mul basic block. This will prevent pmuldq/pmuludq from being
formed at all.

This patch teaches shouldSinkOperands to recognize this so
that CodeGenPrepare will clone the extend into the same basic
block as the mul.

Fixes PR51371.

Differential Revision: https://reviews.llvm.org/D107689