aarch64: Check for invalid use arrays in ldp_fusion [PR118320]

aarch64: Check for invalid use arrays in ldp_fusion [PR118320]

As Andrew says in the bugzilla comments, this PR is about a case where
we tried to fuse two stores of x0, one in which x0 was defined and one
in which it was undefined.  merge_access_arrays failed on the conflict,
but the failure wasn't caught.

Normally the hazard detection code would fail if the instructions
had incompatible uses.  However, an undefined use doesn't impose
many restrictions on movements.  I think this is likely to be the
only case where hazard detection isn't enough.

As Andrew notes in bugzilla, it might be possible to allow uses
of defined and undefined values to be merged to the defined value.
But that sounds dangerous in the general case, as an rtl-ssa-level
decision.  We might run the risk of turning conditional UB into
unconditional UB.  And LLVM proves that the definition of "undef"
isn't simple.

This is a backport of
r15-7282-gf559ac896942ffe0e2315d0a4d8b8b517a16d607.

gcc/
	PR rtl-optimization/118320
	* config/aarch64/aarch64-ldp-fusion.cc (ldp_bb_info::fuse_pair):
	Commonize the merge of input_uses and return early if it
	fails.

gcc/testsuite/
	PR rtl-optimization/118320
	* g++.dg/torture/pr118320.C: New test.

Co-authored-by: Alex Coplan <alex.coplan@arm.com>