Remove dead code related to VEC_COND_EXPR expansion from ISEL

ISEL was introduced to translate vector comparison and vector
condition combinations back to internal function calls mapping to
one of the vcond[u][_eq][_mask] and vec_cmp[_eq] optabs.  With
removing the legacy non-mask vcond expanders we now rely on all
vector comparisons and vector conditions to be directly expandable.
The following keeps the intermediate internal function rewrite
given gimple_expand_vec_cond_expr still performs some optimizations
which eventually should move to vector lowering or match.pd, but
simplifies it down to always expand VEC_COND_EXPR to .VCOND_MASK.

	* gimple-isel.cc (gimple_expand_vec_cond_expr): If not
	simplifying or lowering, always expand to .VCOND_MASK.
	(pass_gimple_isel::execute): Simplify.