[X86] X86FixupVectorConstants - load+sign-extend vector constants that can be stored in a truncated form (#79815)

Reduce the size of the vector constant by storing it in the constant pool in a truncated form, and sign-extend it as part of the load.

I've extended the existing FixupConstant functionality to support these sext constant rebuilds - we still select the smallest stored constant entry and prefer vzload/broadcast/vextload for same bitwidth to avoid domain flips.

I intend to add the matching load+zero-extend handling in a future PR, but that requires some alterations to the existing MC shuffle comments handling first.

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