Use CASE_CONVERT in a few more cases

This uses CASE_CONVERT more which eases eventual removal of NOP_EXPR.

2022-04-29  Richard Biener  <rguenther@suse.de>

gcc/cp/
	* constexpr.cc (fold_simple_1): Use CASE_CONVERT.
	* cp-gimplify.cc (cp_fold): Likewise.
	* pt.cc (tsubst_copy): Likewise.

gcc/
	* dojump.cc (do_jump): Use CASE_CONVERT.
	* tree-ssa-dom.cc (edge_info::derive_equivalences): Likewise.