Improve do_store_flag for comparing single bit against that bit

This is a case which I noticed while working on the previous patch.
Sometimes we end up with `a == CST` instead of comparing against 0.
This happens in the following code:
```
unsigned f(unsigned t)
{
  if (t & ~(1<<30)) __builtin_unreachable();
  t ^= (1<<30);
  return t != 0;
}
```

We should handle the case where the nonzero bits is the same as the
comparison operand.

Changes from v1:
* v2: Updated for the bit extraction changes.

OK? Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* expr.cc (do_store_flag): Improve for single bit testing
	not against zero but against that single bit.