aarch64: Fix up uses of mem following stp insert [PR113070]

As the PR shows (specifically #c7) we are missing updating uses of mem
when inserting an stp in the aarch64 load/store pair fusion pass.  This
patch fixes that.

RTL-SSA has a simple view of memory and by default doesn't allow stores
to be re-ordered w.r.t. other stores.  In the ldp fusion pass, we do our
own alias analysis and so can re-order stores over other accesses when
we deem this is safe.  If neither store can be re-purposed (moved into
the required position to form the stp while respecting the RTL-SSA
constraints), then we turn both the candidate stores into "tombstone"
insns (logically delete them) and insert a new stp insn.

As it stands, we implement the insert case separately (after dealing
with the candidate stores) in fuse_pair by inserting into the middle of
the vector of changes.  This is OK when we only have to insert one
change, but with this fix we would need to insert the change for the new
stp plus multiple changes to fix up uses of mem (note the number of
fix-ups is naturally bounded by the alias limit param to prevent
quadratic behaviour).  If we kept the code structured as is and inserted
into the middle of the vector, that would lead to repeated moving of
elements in the vector which seems inefficient.  The structure of the
code would also be a little unwieldy.

To improve on that situation, this patch introduces a helper class,
stp_change_builder, which implements a state machine that helps to build
the required changes directly in program order.  That state machine is
reponsible for deciding what changes need to be made in what order, and
the code in fuse_pair then simply follows those steps.

Together with the fix in the previous patch for installing new defs
correctly in RTL-SSA, this fixes PR113070.

We take the opportunity to rename the function decide_stp_strategy to
try_repurpose_store, as that seems more descriptive of what it actually
does, since stp_change_builder is now responsible for the overall change
strategy.

gcc/ChangeLog:

	PR target/113070
	* config/aarch64/aarch64-ldp-fusion.cc
	(struct stp_change_builder): New.
	(decide_stp_strategy): Reanme to ...
	(try_repurpose_store): ... this.
	(ldp_bb_info::fuse_pair): Refactor to use stp_change_builder to
	construct stp changes.  Fix up uses when inserting new stp insns.