LRA: Don't emit move for substituted CONSTATNT_P operand [PR116170]

LRA: Don't emit move for substituted CONSTATNT_P operand [PR116170]

Commit r15-2084 exposes one ICE in LRA.  Firstly, before
r15-2084 KFmode has 126 bit precision while V1TImode has 128
bit precision, so the subreg (subreg:V1TI (reg:KF 131) 0) is
paradoxical_subreg_p, which stops some passes from doing
some optimization.  After r15-2084, KFmode has the same mode
precision as V1TImode, passes are able to optimize more, but
it causes this ICE in LRA as described below:

For insn 106 (set (mem:V1TI ...) (subreg:V1TI (reg:KF 133) 0)),
which matches pattern

(define_insn "*vsx_le_perm_store_<mode>"
  [(set (match_operand:VSX_LE_128 0 "memory_operand" "=Z,Q")
        (match_operand:VSX_LE_128 1 "vsx_register_operand" "+wa,r"))]
  "!BYTES_BIG_ENDIAN && TARGET_VSX && !TARGET_P9_VECTOR
   && !altivec_indexed_or_indirect_operand (operands[0], <MODE>mode)"
  "@
   #
   #"
  [(set_attr "type" "vecstore,store")
   (set_attr "length" "12,8")
   (set_attr "isa" "<VSisa>,*")])

LRA makes equivalence substitution on r133 with const double
(const_double:KF 0.0), selects alternative 0 and fixes up
operand 1 for constraint "wa", because operand 1 is OP_INOUT,
so it considers assigning back to it as well, that is:

  lra_emit_move (type == OP_INOUT ? copy_rtx (old) : old, new_reg);

But because old has been changed to const_double in equivalence
substitution, the move is actually assigning to const_double,
which is invalid and cause ICE.

Considering reg:KF 133 is equivalent with (const_double:KF 0.0)
even though this operand is OP_INOUT, IMHO there should not be
any following uses of reg:KF 133, otherwise it doesn't have the
chance to be equivalent to (const_double:KF 0.0).  So this patch
is to guard the lra_emit_move with !CONSTANT_P to exclude such
case.

	PR rtl-optimization/116170

gcc/ChangeLog:

	* lra-constraints.cc (curr_insn_transform): Don't emit move back to
	old operand if it's CONSTANT_P.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/pr116170.c: New test.