Undo multi-word optional reloads correctly

Unlike e.g. remove_inheritance_pseudos, undo_optional_reloads didn't
deal with subregs, so instead of removing multi-word moves, it
replaced the reload pseudo with the original pseudo.  Besides the
redundant move, that retained the clobber of the dest, that starts a
multi-word move.  After the remap, the sequence that should have
become a no-op move starts by clobbering the original pseudo and then
moving its pieces onto themselves.  The problem is the clobber: it
makes earlier sets of the original pseudo to be regarded as dead: if
the optional reload sequence was an output reload, the insn for which
the output reload was attempted may be regarded as dead and deleted.

I've arranged for undo_optional_reloads to accept SUBREGs and use
get_regno, like remove_inheritance_pseudo, adjusted its insn-removal
loop to tolerate iterating over a removed clobber, and added logic to
catch any left-over reload clobbers that could trigger the problem.


for  gcc/ChangeLog

	* lra-constraints.cc (undo_optional_reloads): Recognize and
	drop insns of multi-word move sequences, tolerate removal
	iteration on an already-removed clobber, and refuse to
	substitute original pseudos into clobbers.