recog: Avoid validate_change shortcut for groups [PR115782]

recog: Avoid validate_change shortcut for groups [PR115782]

In this PR, due to the -f flags, we ended up with:

bb1:  r10=r10
...
bb2:  r10=r10
...
bb3:  ...=r10

with bb1->bb2 and bb1->bb3.

late-combine successfully combined the bb1->bb2 def-use and set
the insn code to NOOP_MOVE_INSN_CODE.  The bb1->bb3 combination
then failed for... reasons.  At this point, everything should have
been rewound to its original state.

However, substituting r10=r10 into r10=r10 gives r10=r10, and
validate_change had an early-out for no-op rtl changes.  This meant
that validate_change did not register a change for the bb2 insn and
so did not save its old insn code.  The NOOP_MOVE_INSN_CODE therefore
persisted even after the attempt had been rewound.

IMO it'd be too cumbersome and error-prone to expect all users of
validate_change to be aware of this possibility.  If code is using
validate_change with in_group=1, I think it has a reasonable expectation
that a change will be registered and that the insn code will be saved
(and restored on cancel).  This patch therefore limits the shortcut
to the !in_group case.

gcc/
	PR rtl-optimization/115782
	* recog.cc (validate_change_1): Suppress early exit for no-op
	changes that are part of a group.

gcc/testsuite/
	PR rtl-optimization/115782
	* gcc.dg/pr115782.c: New test.