[X86] Premature replacement of TRUNCATE with PACKSS/PACKUS patterns are preventing generic combines

[RKSimon]

The combineVectorTruncation / combineVectorSignBitsTruncation combines are converting generic truncate nodes to target specific packss/packus nodes very early, mainly as a way to retain these patterns before type legalization. But this prevents the use of generic combines that use truncate.

combineVectorTruncation / combineVectorSignBitsTruncation need to be removed and we need better lowering of truncate nodes from illegal result/operand types.

This is very obvious with https://reviews.llvm.org/D152928

[llvmbot]

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