lra: Replace subregs in bare uses & clobbers [PR108681]

lra: Replace subregs in bare uses & clobbers [PR108681]

In this PR we had a write to one vector of a 4-vector tuple.
The vector had mode V1DI, and the target doesn't provide V1DI
moves, so this was converted into:

    (clobber (subreg:V1DI (reg/v:V4x1DI 92 [ b ]) 24))

followed by a DImode move.  (The clobber isn't really necessary
or helpful for a single word, but would be for wider moves.)

The subreg in the clobber survived until after RA:

    (clobber (subreg:V1DI (reg/v:V4x1DI 34 v2 [orig:92 b ] [92]) 24))

IMO this isn't well-formed.  If a subreg of a hard register simplifies
to a hard register, it should be replaced by the hard register.  If the
subreg doesn't simplify, then target-independent code can't be sure
which parts of the register are affected and which aren't.  A clobber
of such a subreg isn't useful and (again IMO) should just be removed.
Conversely, a use of such a subreg is effectively a use of the whole
inner register.

LRA has code to simplify subregs of hard registers, but it didn't
handle bare uses and clobbers.  The patch extends it to do that.

One question was whether the final_p argument to alter_subregs
should be true or false.  True is IMO dangerous, since it forces
replacements that might not be valid from a dataflow perspective,
and uses and clobbers only exist for dataflow.  As said above,
I think the correct way of handling a failed simplification would
be to delete clobbers and replace uses of subregs with uses of
the inner register.  But I didn't want to write untested code
to do that.

In the PR, the clobber caused an infinite loop in DCE, because
of a disagreement about what effect the clobber had.  But for
the reasons above, I think that was GIGO rather than a bug in
DF or DCE.

gcc/
	PR rtl-optimization/108681
	* lra-spills.cc (lra_final_code_change): Extend subreg replacement
	code to handle bare uses and clobbers.

gcc/testsuite/
	PR rtl-optimization/108681
	* gcc.target/aarch64/pr108681.c: New test.