[Bug target/63949] Aarch64 instruction combiner does not optimize subsi_sxth function as expected (gcc.target/aarch64/extend.c fails)

[rearnsha at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63949

--- Comment #3 from Richard Earnshaw <rearnsha at gcc dot gnu.org> ---
make_extraction is unable to generate bit-field extractions in more than one
mode.  This causes the extractions that it does generate to be wrapped in
subregs when SImode results are wanted.

Ideally, we should teach make_extraction to be more sensible, but I'm not sure
what the impact of that would be on other ports that really can only support
one mode for bit-field extracts.