[X86] Generalize...

[X86] Generalize combineVectorTruncationWithPACKUS/combineVectorTruncationWithPACKSS and reuse in LowerTRUNCATE

Rename combineVectorTruncationWithPACK* to truncateVectorWithPACK* and split the operands so LowerTRUNCATE can more easily use them.

Noticed while investigating some regressions in D152928 due to us trying to truncate to PACKUS/PACKSS instructions too early