GlobalISel: Combine g_extract with g_merge_values
Try to use the underlying source registers.
This enables legalization in more cases where some irregular
operations are widened and others narrowed.
This seems to make the test_combines_2 AArch64 test worse, since the
MERGE_VALUES has multiple uses. Since this should be required for
legalization, a hasOneUse check is probably inappropriate (or maybe
should only be used if the merge is legal?).
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353121
91177308-0d34-0410-b5e6-
96231b3b80d8