aarch64: Detect word-level modification in early-ra [PR118184]

aarch64: Detect word-level modification in early-ra [PR118184]

REGMODE_NATURAL_SIZE is set to 64 bits for everything except
VLA SVE modes.  This means that it's possible to modify (say)
the highpart of a TI pseudo or a V2DI pseudo independently
of the lowpart.  Modifying such highparts requires a reload
if the highpart ends up in the upper 64 bits of an FPR,
since RTL semantics do not allow the highpart of a single
hard register to be modified independently of the lowpart.

early-ra missed a check for this case, which meant that it
effectively treated an assignment to (subreg:DI (reg:TI R) 0)
as an assignment to the whole of R.

gcc/
	PR target/118184
	* config/aarch64/aarch64-early-ra.cc (allocno_assignment_is_rmw):
	New function.
	(early_ra::record_insn_defs): Mark the live range information as
	untrustworthy if an assignment would change part of an allocno
	but preserve the rest.

gcc/testsuite/
	* gcc.dg/torture/pr118184.c: New test.