[X86] Optimize sign extends on index operand to gather/scatter to not sign extend past i32.

The gather instruction will implicitly sign extend to the pointer width, we don't need to further extend it. This can prevent unnecessary splitting in some cases.

There's still an issue that lowering on non-VLX can introduce another sign extend that doesn't get combined with shifts from a lowered sign_extend_inreg.

llvm-svn: 321152