Remove unreachable gcc_unreachable () at the end of functions

Remove unreachable gcc_unreachable () at the end of functions

It seems to be a style to place gcc_unreachable () after a
switch that handles all cases with every case returning.
Those are unreachable (well, yes!), so they will be elided
at CFG construction time and the middle-end will place
another __builtin_unreachable "after" them to note the
path doesn't lead to a return when the function is not declared
void.

So IMHO those explicit gcc_unreachable () serve no purpose,
if they could be replaced by a comment.  But since all cases
cover switches not handling a case or not returning will
likely cause some diagnostic to be emitted which is better
than running into an ICE only at runtime.

2021-11-24  Richard Biener  <rguenther@suse.de>

	* tree.h (reverse_storage_order_for_component_p): Remove
	spurious gcc_unreachable.
	* cfganal.c (dfs_find_deadend): Likewise.
	* fold-const-call.c (fold_const_logb): Likewise.
	(fold_const_significand): Likewise.
	* gimple-ssa-store-merging.c (lhs_valid_for_store_merging_p):
	Likewise.

gcc/c-family/
	* c-format.c (check_format_string): Remove spurious
	gcc_unreachable.