Fix expansion of `(a & 2) != 1`

I had a thinko in r14-1600-ge60593f3881c72a96a3fa4844d73e8a2cd14f670
where we would remove the `& CST` part if we ended up not calling
expand_single_bit_test.
This fixes the problem by introducing a new variable that will be used
for calling expand_single_bit_test.
As afar as I know this can only show up when disabling optimization
passes as this above form would have been optimized away.

Committed as obvious after a bootstrap/test on x86_64-linux-gnu.

	PR middle-end/111863

gcc/ChangeLog:

	* expr.cc (do_store_flag): Don't over write arg0
	when stripping off `& POW2`.

gcc/testsuite/ChangeLog:

	* gcc.c-torture/execute/pr111863-1.c: New test.