gimple: Don't assert that switch has nondefault cases during lowering [PR120080]

gimple: Don't assert that switch has nondefault cases during lowering [PR120080]

I have mistakenly assumed that switch lowering cannot encounter a switch
with zero clusters.  This patch removes the relevant assert and instead
gives up bit-test lowering when this happens.

	PR tree-optimization/120080

gcc/ChangeLog:

	* tree-switch-conversion.cc (bit_test_cluster::find_bit_tests):
	Replace assert with return.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/pr120080.c: New test.

Signed-off-by: Filip Kastl <fkastl@suse.cz>