gimple-match: Make code_helper conversions explicit

gimple-match: Make code_helper conversions explicit

code_helper provides conversions to tree_code and combined_fn.
Now that the codebase is C++11, we can mark these conversions as
explicit.  This avoids accidentally using code_helpers with
functions that take tree_codes, which would previously entail
a hidden unchecked conversion.

gcc/
	* gimple-match.h (code_helper): Provide == and != overloads.
	(code_helper::operator tree_code): Make explicit.
	(code_helper::operator combined_fn): Likewise.
	* gimple-match-head.c (convert_conditional_op): Use explicit
	conversions where necessary.
	(gimple_resimplify1, gimple_resimplify2, gimple_resimplify3): Likewise.
	(maybe_push_res_to_seq, gimple_simplify): Likewise.
	* gimple-fold.c (replace_stmt_with_simplification): Likewise.