This is restricted to single use loads, which if we fold to sextloads we can find more optimal addressing modes on AArch64.
LGTM with a couple nits on comments
(I can totally imagine typing "InReg" all of the time instead of "Inreg" and getting annoyed, but that may be a personal fault.)
I don't think we need Doxygen comments here?
Comment is weird here?
I don't think we do this everywhere in the combiner. Should we be doing this everywhere? (We should probably be doing this everywhere.)