Introduce can_vcond_compare_p function

Introduce can_vcond_compare_p function

z13 supports only non-signaling vector comparisons.  This means we
cannot vectorize LT, LE, GT, GE and LTGT when compiling for z13.
However, we cannot express this restriction today: the code only checks
whether vcond$a$b optab exists, but this does not say anything about the
operation.

Introduce a function that checks whether back-end supports vector
comparisons with individual rtx codes by matching vcond expander's third
argument with a fake comparison with the corresponding rtx code.

gcc/ChangeLog:

2019-10-07  Ilya Leoshkevich  <iii@linux.ibm.com>

	PR target/77918
	* optabs-tree.c (vcond_icode_p): New function.
	(vcond_eq_icode_p): Likewise.
	(expand_vec_cond_expr_p): Use vcond_icode_p and
	vcond_eq_icode_p.
	* optabs.c (can_vcond_compare_p): New function.
	* optabs.h (can_vcond_compare_p): Likewise.

From-SVN: r276660