cris: try to generate zero-based comparisons

cris: try to generate zero-based comparisons

* config/cris/cris.c (cris_reduce_compare): New function.
* config/cris/cris-protos.h  (cris_reduce_compare): Add prototype.
* config/cris/cris.md ("cbranch<mode>4", "cbranchdi4", "cstoredi4")
(cstore<mode>4"): Apply cris_reduce_compare in expanders.

The decc0ration work of the CRIS port made me look closer at the
code for trivial comparisons, as in the condition for branches
and conditional-stores, like in:

void g(short int a, short int b)
{
  short int c = a + b;

  if (c >= 0)
    foo ();
}

At -O2, the cc0 version of the CRIS port has an explicit
*uneliminated* compare instruction ("cmp.w -1,$r10") instead of
an (eliminated) compare against 0 (which below I'll call a
zero-compare).  This for the CRIS-cc0 version, but I see this
also for a much older gcc, at 4.7.  For the decc0rated port, the
compare *is* a test against 0, eventually eliminated.  To wit,
for cc0 (mind the delay-slot):

_g:
	subq 4,$sp
	add.w $r11,$r10
	cmp.w -1,$r10
	ble .L9
	move $srp,[$sp]

	jsr _foo
.L9:
	jump [$sp+]

The compare instruction is expected to be eliminated, i.e.  the
following diff to the above is desired, modulo the missing
sibling call, which corresponds to what I get from 4.7 and for
the decc0rated port:

!--- a	Wed Feb  5 15:22:27 2020
!+++ b	Wed Feb  5 15:22:51 2020
!@@ -1,8 +1,7 @@
! _g:
!	 subq 4,$sp
!	 add.w $r11,$r10
!-	cmp.w -1,$r10
!-	ble .L9
!+	bmi .L9
!	 move $srp,[$sp]
!
!	 jsr _foo

Tracking this difference, I see that for both cc0-CRIS and the
decc0rated CRIS, the comparison actually starts out as a compare
against -1 at "expand" time, but is transformed for decc0rated
CRIS to a zero-compare in "cse1".

For CRIS-cc0 "cse1" does try to replace the compare with a
zero-compare, but fails because at the same time it tries to
replace the c operand with (a + b).  Or some such; it fails and
no other pass succeeds.  I was not into fixing cc0-handling in
core gcc, so I didn't look closer.

BTW, at first, I was a bit surprised to see that for compares
against a constant, a zero-compare is not canonical RTX for
*all* conditions, and that instead only a subset of all RTX
conditions against a constant are canonical, transforming one
condition to the canonical one by adding 1 or -1 to the
constant.  It does makes sense at a closer look, but still not
so much when emitting RTL.

There are several places that mention in comments that emitting
RTX as zero-compare is preferable, but nothing is done about it.
Some generic code instead seems confused that the *target* is
helped by seeing canonical RTX, or perhaps it (its authors) like
me, confused about what a canonical comparison is.  For example,
prepare_cmp_insn calls canonicalize_comparison last before
emitting the actual instructions.  I see most ports for various
port-specific reasons does their own massaging in their cbranch
and cstore expanders.  Still, the suboptimal compares *should*
be fixed at expand time; better start out right than just
relying on later optimizations.

This kind of change is not acceptable in the current gcc
development stage, at least as a change in generic code.
However, it's problematic enough that I chose to fix this right
now in the CRIS port.  For that, I claim a possibly
long-standing regression.  After this, code before and after
decc0ration is similar enough that I can spot
compare-elimination-efforts and apply regression test-cases
without them drowning in cc0-specific xfailing.

I hope to eventually lift out cris_reduce_compare (renamed) into
say expmed.c, called in e.g. emit_store_flag_1 (replacing the
in-line code) and prepare_cmp_insn.  Later.