hardcmp: split before dispatch edge

If we harden a compare at the end of a block with an edge to the
abnormal dispatch block, it won't have a single successor.  Arrange to
split the block at its final stmt so as to have a single succ.


for  gcc/ChangeLog

	PR middle-end/104975
	* gimple-harden-conditionals.cc
	(pass_harden_compares::execute): Force split in case of
	multiple edges.

for  gcc/testsuite/ChangeLog

	PR middle-end/104975
	* gcc.dg/pr104975.c: New.