Introduce can_vec_cmp_compare_p

Introduce can_vec_cmp_compare_p

This is the same as dcd2ca63ec5c ("Introduce can_vcond_compare_p
function"), but for vec_cmp.  The reason it's needed is that since
5d9ade39b872 ("IBM Z: Fix PR97326: Enable fp compares in vec_cmp")
and 4acba4859013 ("IBM Z: Restrict vec_cmp<m><n> on z13") s390's vec_cmp
expander advertises that it supports floating point comparisons except
signaling ones on z13, but the common code ignores the latter
restriction.

gcc/ChangeLog:

2020-11-25  Ilya Leoshkevich  <iii@linux.ibm.com>

	* optabs-tree.c (vec_cmp_icode_p): New function.
	(vec_cmp_eq_icode_p): New function.
	(expand_vec_cmp_expr_p): Use vec_cmp_icode_p and
	vec_cmp_eq_icode_p.
	(vcond_icode_p): Use get_rtx_code_1, just to be uniform with
	vec_cmp_icode_p.
	* optabs.c (unsigned_optab_p): New function.
	(insn_predicate_matches_p): New function.
	(can_vec_cmp_compare_p): New function.
	(can_vcond_compare_p): Use unsigned_optab_p and
	insn_predicate_matches_p.
	(get_rtx_code): Use get_rtx_code_1.
	(get_rtx_code_1): Version of get_rtx_code that returns UNKNOWN
	instead of asserting.
	* optabs.h (can_vec_cmp_compare_p): New function.
	(get_rtx_code_1): New function.