aarch64: Fix ICE when op2 is zero for SVE2 saturating add intrinsics.

When op2 in SVE2 saturating add intrinsics (svuqadd, svsqadd) is a zero
vector and predication is _z, an ICE in vregs occurs, e.g. for

svuint8_t foo (svbool_t pg, svuint8_t op1)
{
    return svsqadd_u8_z (pg, op1, svdup_s8 (0));
}

The insn failed to match the pattern (aarch64-sve2.md):

;; Predicated binary operations with no reverse form, merging with zero.
;; At present we don't generate these patterns via a cond_* optab,
;; so there's no correctness requirement to handle merging with an
;; independent value.
(define_insn_and_rewrite "*cond_<sve_int_op><mode>_z"
  [(set (match_operand:SVE_FULL_I 0 "register_operand")
	(unspec:SVE_FULL_I
	  [(match_operand:<VPRED> 1 "register_operand")
	   (unspec:SVE_FULL_I
	     [(match_operand 5)
	      (unspec:SVE_FULL_I
		[(match_operand:SVE_FULL_I 2 "register_operand")
		 (match_operand:SVE_FULL_I 3 "register_operand")]
		SVE2_COND_INT_BINARY_NOREV)]
	     UNSPEC_PRED_X)
	   (match_operand:SVE_FULL_I 4 "aarch64_simd_imm_zero")]
	  UNSPEC_SEL))]
  "TARGET_SVE2"
  {@ [ cons: =0 , 1   , 2 , 3  ]
     [ &w       , Upl , 0 , w  ] movprfx\t%0.<Vetype>, %1/z, %0.<Vetype>\;<sve_int_op>\t%0.<Vetype>, %1/m, %0.<Vetype>, %3.<Vetype>
     [ &w       , Upl , w , w  ] movprfx\t%0.<Vetype>, %1/z, %2.<Vetype>\;<sve_int_op>\t%0.<Vetype>, %1/m, %0.<Vetype>, %3.<Vetype>
  }
  "&& !CONSTANT_P (operands[5])"
  {
    operands[5] = CONSTM1_RTX (<VPRED>mode);
  }
  [(set_attr "movprfx" "yes")]
)

because operands[3] and operands[4] were both expanded into the same register
operand containing a zero vector by define_expand "@cond_<sve_int_op><mode>".

This patch fixes the ICE by making a case distinction in
function_expander::use_cond_insn that uses add_fixed_operand if
fallback_arg == CONST0_RTX (mode), and otherwise add_input_operand (which was
previously the default and allowed the expansion of the zero-vector
fallback_arg to a register operand).

The patch was bootstrapped and tested on aarch64-linux-gnu, no regression.
OK for trunk?

Alex Coplan pointed out in the bugzilla ticket that this ICE goes back
to GCC 10. Shall we backport?

Signed-off-by: Jennifer Schmitz <jschmitz@nvidia.com>
Co-authored by: Richard Sandiford <rdsandiford@googlemail.com>

gcc/
	PR target/121599
	* config/aarch64/aarch64-sve-builtins.cc
	(function_expander::use_cond_insn): Use add_fixed_operand if
	fallback_arg == CONST0_RTX (mode).

gcc/testsuite/
	PR target/121599
	* gcc.target/aarch64/sve2/pr121599.c: New test.