[X86] Add test case to show missed opportunity to shrink a constant index to a...

[X86] Add test case to show missed opportunity to shrink a constant index to a gather in order to avoid splitting.

Also add a test case for an index that could be shrunk, but
would create a narrow type. We can go ahead and do it we just
need to be before type legalization.

Similar test cases for scatter as well.

llvm-svn: 373290