Improve do_store_flag for single bit comparison against 0

While working something else, I noticed we could improve
the following function code generation:
```
unsigned f(unsigned t)
{
  if (t & ~(1<<30)) __builtin_unreachable();
  return t != 0;
}
```
Right know we just emit a comparison against 0 instead
of just a shift right by 30.
There is code in do_store_flag which already optimizes
`(t & 1<<30) != 0` to `(t >> 30) & 1` (using bit extraction if available).
This patch extends it to handle the case where we know t has a nonzero
of just one bit set.

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

OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.

gcc/ChangeLog:

	* expr.cc (do_store_flag): Extend the one bit checking case
	to handle the case where we don't have an and but rather still
	one bit is known to be non-zero.