pair-fusion: A couple of fixes for sp updates [PR118429]

The PR showed two issues with pair-fusion.  The first is that the pass
treated stack pointer deallocations as ordinary register updates, and so
might move them earlier than another stack access (through a different
base register) that doesn't alias the pair candidate.

The simplest fix for that seems to be to prevent the stack deallocation
from being moved.  This part might (or might not) be a latent source of
wrong code and so worth backporting in some form.  (The patch as-is
won't work for GCC 14.)

The second issue only started with r15-6551, which added a memory
write to stack allocations and deallocations.  We should use the
existing tombstone mechanism to preserve the associated memory
definition.  (Deleting definitions immediately would have quadratic
complexity in the worst case.)

gcc/
	PR rtl-optimization/118429
	* pair-fusion.cc (latest_hazard_before): Add an extra parameter
	to say whether the instruction is a load or a store.  If the
	instruction is not a load or store and has memory side effects,
	prevent it from being moved earlier.
	(pair_fusion::find_trailing_add): Update call accordingly.
	(pair_fusion_bb_info::fuse_pair): If the trailng addition had
	a memory side-effect, use a tombstone to preserve it.

gcc/testsuite/
	PR rtl-optimization/118429
	* gcc.c-torture/compile/pr118429.c: New test.