Remove more stray returns and gcc_unreachable ()s

This removes more cases that appear when bootstrap with
-Wunreachable-code-return progresses.

2021-11-29  Richard Biener  <rguenther@suse.de>

	* config/i386/i386.c (ix86_shift_rotate_cost): Remove
	unreachable return.
	* tree-chrec.c (evolution_function_is_invariant_rec_p):
	Likewise.
	* tree-if-conv.c (if_convertible_stmt_p): Likewise.
	* tree-ssa-pre.c (fully_constant_expression): Likewise.
	* tree-vrp.c (operand_less_p): Likewise.
	* reload.c (reg_overlap_mentioned_for_reload_p): Remove
	unreachable gcc_unreachable ().
	* sel-sched-ir.h (bb_next_bb): Likewise.
	* varasm.c (compare_constant): Likewise.

gcc/cp/
	* logic.cc (cnf_size_r): Remove unreachable and inconsistently
	placed gcc_unreachable ()s.
	* pt.c (iterative_hash_template_arg): Remove unreachable
	gcc_unreachable and return.

gcc/fortran/
	* target-memory.c (gfc_element_size): Remove unreachable return.

gcc/objc/
	* objc-act.c (objc_build_setter_call): Remove unreachable
	return.

libcpp/
	* charset.c (convert_escape): Remove unreachable break.