Improve backward threading with switches.

Improve backward threading with switches.

We've been essentially using find_taken_edge_switch_expr() in the
backward threader, but this is suboptimal because said function only
works with singletons.  VRP has a much smarter find_case_label_range
that works with ranges.

Tested on x86-64 Linux with:

	a) Bootstrap & regtests.

	b) Verifying we get more threads than before.

	c) Asserting that the new code catches everything the old one
	code caught (over a set of bootstrap .ii files).

gcc/ChangeLog:

	* tree-ssa-threadbackward.c
	(back_threader::find_taken_edge_switch): Use find_case_label_range
	instead of find_taken_edge.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/vrp106.c: Adjust for threading.
	* gcc.dg/tree-ssa/vrp113.c: Same.