ira: tie output allocnos for fused instruction pairsHEADtrunkmaster

Some of the instruction pairs recognized as fusible by a preceding
invocation of the dep_fusion pass require that both components of a pair
have the same hard register output for the fusion to work in hardware.
(An example of this would be a multiply-add operation, or a zero-extract
operation composed of two shifts.)

For all such pairs, the following conditions will hold:
  (a) Both insns are single_sets
  (b) Both insns have a register destination
  (c) The pair has been marked as fusible by setting the second insn's
SCHED_GROUP flag
  (d) Additionally, post-RA, both instructions' destination regnos are
equal

(All of these conditions are encapsulated in the newly created
single_output_fused_pair_p () predicate.)

During IRA, if conditions (a)-(c) above hold, we need to tie the two
instructions' destination allocnos together so that they are allocated
to the same hard register.  We do this in add_insn_allocno_copies () by
adding a constraint conflict to the output operands of the two
instructions.

gcc/ChangeLog:

	* ira-conflicts.cc (add_insn_allocno_copies): Handle fused insn pairs.
	* rtl.h (single_output_fused_pair_p): Declare new function.
	* rtlanal.cc (single_output_fused_pair_p): Define it.

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