[lra] recompute ranges upon disabling fp2sp elimination [PR120424]

[lra] recompute ranges upon disabling fp2sp elimination [PR120424]

If the frame size grows to nonzero, arm_frame_pointer_required may
flip to true under -fstack-clash-protection -fnon-call-exceptions, and
that may disable the fp2sp elimination part-way through lra.

If pseudos had got assigned to the frame pointer register before that,
they have to be spilled, and that requires complete live range
information.  If !lra_reg_spill_p, lra_spill won't have live ranges
for such pseudos, and they could end up sharing spill slots with other
pseudos whose live ranges actually overlap.

This affects at least Ada.Strings.Wide_Superbounded.Super_Insert and
.Super_Replace_Slice in libgnat/a-stwisu.adb, when compiled with -O2
-fstack-clash-protection -march=armv7 (implied Thumb2), causing
acats-4's cdd2a01 to fail.

Recomputing live ranges including registers may renumber and compress
points, so we have to recompute the aggregated live ranges for
already-assigned spill slots as well.

As a safety net, reject empty live ranges when computing slot sharing.

for  gcc/ChangeLog

	PR rtl-optimization/120424
	* lra-eliminations.cc (lra_update_fp2sp_elimination):
	Compute complete live ranges and recompute slots' live ranges
	if needed.
	* lra-lives.cc (lra_reset_live_range_list): New.
	(lra_complete_live_ranges): New.
	* lra-spills.cc (assign_spill_hard_regs): Reject empty live
	ranges.
	(add_pseudo_to_slot): Likewise.
	(lra_recompute_slots_live_ranges): New.
	* lra-int.h (lra_reset_live_range_list): Declare.
	(lra_complete_live_ranges): Declare.
	(lra_recompute_slots_live_ranges): Declare.