Avoid some -Wunreachable-code-ctrl

Avoid some -Wunreachable-code-ctrl

This cleans up unreachable code diagnosed by -Wunreachable-code-ctrl.
It largely follows the previous series but discovers a few extra
cases, namely dead code after break or continue or loops without
exits.

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

gcc/c/
	* gimple-parser.c (c_parser_gimple_postfix_expression):
	avoid unreachable code after break.

gcc/
	* cfgrtl.c (skip_insns_after_block): Refactor code to
	be more easily readable.
	* expr.c (op_by_pieces_d::run): Remove unreachable
	assert.
	* sched-deps.c (sched_analyze): Remove unreachable
	gcc_unreachable.
	* sel-sched-ir.c (in_same_ebb_p): Likewise.
	* tree-ssa-alias.c (nonoverlapping_refs_since_match_p):
	Remove unreachable code.
	* tree-vect-slp.c (vectorize_slp_instance_root_stmt):
	Refactor to avoid unreachable loop iteration.
	* tree.c (walk_tree_1): Remove unreachable break.
	* vec-perm-indices.c (vec_perm_indices::series_p): Remove
	unreachable return.

gcc/cp/
	* parser.c (cp_parser_postfix_expression): Remove
	unreachable code.
	* pt.c (tsubst_expr): Remove unreachable breaks.

gcc/fortran/
	* frontend-passes.c (gfc_expr_walker): Remove unreachable
	break.
	* scanner.c (skip_fixed_comments): Remove unreachable
	gcc_unreachable.
	* trans-expr.c (gfc_expr_is_variable): Refactor to make
	control flow more obvious.