cse: Use simplify_replace_fn_rtx to process notes [PR94740]

cse: Use simplify_replace_fn_rtx to process notes [PR94740]

cse_process_notes did a very simple substitution, which in the wrong
circumstances could create non-canonical RTL and invalid MEMs.
Various sticking plasters have been applied to cse_process_notes_1
to handle cases like ZERO_EXTEND, SIGN_EXTEND and UNSIGNED_FLOAT,
but I think this PR is a plaster too far.

The code is trying hard to avoid creating unnecessary rtl, which of
course is a good thing.  If we continue to do that, then we can end
up changing subexpressions while keeping the containing rtx.
This in turn means that validate_change will be a no-op on the
containing rtx, even if its contents have changed.  So in these
cases we have to apply validate_change to the individual subexpressions.

On the other hand, if we always apply validate_change to the
individual subexpressions, we'll end up calling validate_change
on something before it has been simplified and canonicalised.
And that's one of the situations we're trying to avoid.

There might be a middle ground in which we queue the validate_changes
as part of a group, and so can cancel the pending validate_changes
for subexpressions if there's a change in the outer expression.
But that seems even more ad-hoc than the current code.
It would also be quite an invasive change.

I think the best thing is just to hook into the existing
simplify_replace_fn_rtx function, keeping the REG and MEM handling
from cse_process_notes_1 essentially unchanged.  It can generate
more redundant rtl when a simplification takes place, but it has
the advantage of being relative well-used code (both directly
and via simplify_replace_rtx).

2020-04-30  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR rtl-optimization/94740
	* cse.c (cse_process_notes_1): Replace with...
	(cse_process_note_1): ...this new function, acting as a
	simplify_replace_fn_rtx callback to process_note.  Handle only
	REGs and MEMs directly.  Validate the MEM if cse_process_note
	changes its address.
	(cse_process_notes): Replace with...
	(cse_process_note): ...this new function.
	(cse_extended_basic_block): Update accordingly, iterating over
	the register notes and passing individual notes to cse_process_note.