if-to-switch: Support chain with 2 BBs.

if-to-switch: Support chain with 2 BBs.

As seen in the test-case, even 2 BBs can handle interesting
cases covered by a jump table or a bit-test.

gcc/ChangeLog:

	PR tree-optimization/88702
	* gimple-if-to-switch.cc (pass_if_to_switch::execute):
	Require at least 2 BBs.
	* gimple-if-to-switch.cc (find_conditions): Require
	equal precision for low and high of a range.

gcc/testsuite/ChangeLog:

	PR tree-optimization/88702
	* gcc.dg/tree-ssa/if-to-switch-9.c: New test.