middle-end/61747 - conditional move expansion and constants

middle-end/61747 - conditional move expansion and constants

When expanding a COND_EXPR or a VEC_COND_EXPR the x86 backend for
example tries to match FP min/max instructions.  But this only
works when it can see the equality of the comparison and selected
operands.  This breaks in both prepare_cmp_insn and vector_compare_rtx
where the former forces expensive constants to a register and the
latter performs legitimization.  The patch below fixes this in
the caller preserving former equalities.

	PR middle-end/61747
	* internal-fn.cc (expand_vec_cond_optab_fn): When the
	value operands are equal to the original comparison operands
	preserve that equality by re-using the comparison expansion.
	* optabs.cc (emit_conditional_move): When the value operands
	are equal to the comparison operands and would be forced to
	a register by prepare_cmp_insn do so earlier, preserving the
	equality.

	* g++.target/i386/pr61747.C: New testcase.