ira: Try to avoid propagating conflicts

Suppose that:

- an inner loop L contains an allocno A
- L clobbers hard register R while A is live
- A's parent allocno is AP

Previously, propagate_allocno_info would propagate conflict sets up the
loop tree, so that the conflict between A and R would become a conflict
between AP and R (and so on for ancestors of AP).

However, when IRA treats loops as separate allocation regions, it can
decide on a loop-by-loop basis whether to allocate a register or spill
to memory.  Conflicts in inner loops therefore don't need to become
hard conflicts in parent loops.  Instead we can record that using the
“conflicting” registers for the parent allocnos has a higher cost.
In the example above, this higher cost is the sum of:

- the cost of saving R on entry to L
- the cost of keeping the pseudo register in memory throughout L
- the cost of reloading R on exit from L

This value is also a cap on the hard register cost that A can contribute
to AP in general (not just for conflicts).  Whatever allocation we pick
for AP, there is always the option of spilling that register to memory
throughout L, so the cost to A of allocating a register to AP can't be
more than the cost of spilling A.

To take an extreme example: if allocating a register R2 to A is more
expensive than spilling A to memory, ALLOCNO_HARD_REG_COSTS (A)[R2]
could be (say) 2 times greater than ALLOCNO_MEMORY_COST (A) or 100
times greater than ALLOCNO_MEMORY_COST (A).  But this scale factor
doesn't matter to AP.  All that matters is that R2 is more expensive
than memory for A, so that allocating R2 to AP should be costed as
spilling A to memory (again assuming that A and AP are in different
allocation regions).  Propagating a factor of 100 would distort the
register costs for AP.

move_spill_restore tries to undo the propagation done by
propagate_allocno_info, so we need some extra processing there.

gcc/
	PR rtl-optimization/98782
	* ira-int.h (ira_allocno::might_conflict_with_parent_p): New field.
	(ALLOCNO_MIGHT_CONFLICT_WITH_PARENT_P): New macro.
	(ira_single_region_allocno_p): New function.
	(ira_total_conflict_hard_regs): Likewise.
	* ira-build.c (ira_create_allocno): Initialize
	ALLOCNO_MIGHT_CONFLICT_WITH_PARENT_P.
	(ira_propagate_hard_reg_costs): New function.
	(propagate_allocno_info): Use it.  Try to avoid propagating
	hard register conflicts to parent allocnos if we can handle
	the conflicts by spilling instead.  Limit the propagated
	register costs to the cost of spilling throughout the child loop.
	* ira-color.c (color_pass): Use ira_single_region_allocno_p to
	test whether a child and parent allocno can share the same
	register.
	(move_spill_restore): Adjust for the new behavior of
	propagate_allocno_info.

gcc/testsuite/
	* gcc.target/aarch64/reg-alloc-2.c: New test.

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