aarch64: Validate register operands early in ldp fusion pass [PR113062]

aarch64: Validate register operands early in ldp fusion pass [PR113062]

We were missing validation of the candidate register operands in the
ldp/stp pass.  I was relying on recog rejecting such cases when we
formed the final pair insn, but the testcase shows that with
-fharden-conditionals we attempt to combine two insns with asm_operands,
both containing mem rtxes.  This then trips the assert:

gcc_assert (change->new_uses.is_valid ());

in the stp case as we aren't expecting to have (distinct) uses of mem in
the candidate stores.

While doing this I noticed that it seems more natural to have the
initial definition of mem_size closer to its first use in track_access,
so I moved that down.

gcc/ChangeLog:

	PR target/113062
	* config/aarch64/aarch64-ldp-fusion.cc
	(ldp_bb_info::track_access): Punt on accesses with invalid
	register operands, move definition of mem_size closer to its
	first use.

gcc/testsuite/ChangeLog:

	PR target/113062
	* gcc.dg/pr113062.c: New test.