[X86] X86FixupVectorConstants - load+zero vector constants that can be stored...

[X86] X86FixupVectorConstants - load+zero vector constants that can be stored in a truncated form (#80428)

Further develops the vsextload support added in #79815 / b5d35fea - reduces the size of the vector constant by storing it in the constant pool in a truncated form, and zero-extend it as part of the load.