aarch64: Tighten early-ra chain test for wide registers [PR113295]

aarch64: Tighten early-ra chain test for wide registers [PR113295]

Most code in early-ra used is_chain_candidate to check whether we
should chain two allocnos.  This included both tests that matter
for correctness and tests for certain heuristics.

Once that test passes for one pair of allocnos, we test whether
it's safe to chain the containing groups (which might contain
multiple allocnos for x2, x3 and x4 modes).  This test used an
inline test for correctness only, deliberately skipping the
heuristics.  However, this instance of the test was missing
some handling of equivalent allocnos.

This patch fixes things by making is_chain_candidate take a
strictness parameter: correctness only, or correctness + heuristics.
It then makes the group-chaining test use the correctness version
rather than trying to replicate it inline.

gcc/
	PR target/113295
	* config/aarch64/aarch64-early-ra.cc
	(early_ra::test_strictness): New enum.
	(early_ra::is_chain_candidate): Add a strictness parameter to
	control whether only correctness matters, or whether both correctness
	and heuristics should be used.  Handle multiple levels of equivalence.
	(early_ra::find_related_start): Update call accordingly.
	(early_ra::strided_polarity_pref): Likewise.
	(early_ra::form_chains): Likewise.
	(early_ra::try_to_chain_allocnos): Use is_chain_candidate in
	correctness mode rather than trying to inline the test.

gcc/testsuite/
	PR target/113295
	* gcc.target/aarch64/pr113295-2.c: New test.