[X86] X86FixupVectorConstants - add destination register width to...

[X86] X86FixupVectorConstants - add destination register width to rebuildSplatCst/rebuildZeroUpperCst/rebuildExtCst callbacks

As found on #81136 - we aren't correctly handling for cases where the constant pool entry is wider than the destination register width, causing incorrect scaling of the truncated constant for load-extension cases.

This first patch just pulls out the destination register width argument, its still currently driven by the constant pool entry but that will be addressed in a followup.