[X86] combinePMULH - recognise 'cheap' trunctions via PACKS/PACKUS as well as SEXT/ZEXT

combinePMULH currently only truncates vXi32/vXi64 multiplies to PMULHW/PMULUW if the source operands are SEXT/ZEXT instructions for a 'free' truncation.

But we can generalize this to any source operand with sufficient leading sign/zero bits that would allow PACKS/PACKUS to be used as a 'cheap' truncation.

This helps us avoid the wider multiplies, in exchange for truncation on both source operands instead of the result.

Differential Revision: https://reviews.llvm.org/D113371