VEC_COND_EXPR code cleanup

VEC_COND_EXPR code cleanup

This removes now unnecessary special-casings of VEC_COND_EXPRs after
making its first operand a gimple value.

2021-04-14  Richard Biener  <rguenther@suse.de>

	* genmatch.c (lower_cond): Remove VEC_COND_EXPR special-casing.
	(capture_info::capture_info): Likewise.
	(capture_info::walk_match): Likewise.
	(expr::gen_transform): Likewise.
	(dt_simplify::gen_1): Likewise.
	* gimple-match-head.c (maybe_resimplify_conditional_op):
	Remove VEC_COND_EXPR special-casing.
	(gimple_simplify): Likewise.
	* gimple.c (gimple_could_trap_p_1): Adjust.
	* tree-ssa-pre.c (compute_avail): Allow VEC_COND_EXPR
	to participate in PRE.