[X86][SSE] Prefer PACKUS(AND(),AND()) to SHUFFLE(PSHUFB(),PSHUFB()) on pre-AVX2 targets

As discussed on PR31443, we should be trying to use PACKUS for binary truncation patterns to reduce the number of shuffles.

The plan is to support AVX2+ targets once we've worked around PR45315 - we fail to peek through a VBROADCAST_LOAD mask to recognise zero upper bits in a PACKUS pattern.

We should also be able to add support for v8i16 and possibly 256/512-bit vectors as well.