[range-op] Restrict division by power of 2 optimization to positive numbers.

[range-op] Restrict division by power of 2 optimization to positive numbers.

The problem here is that we are transforming a division by a power of
2 into a right shift, and using this to shift the maybe nonzero bits.
This gives the wrong result when the number being divided is negative.

In the testcase we are dividing this by 8:

	[irange] int [-256, -255] NONZERO 0xffffff01

and coming up with:

	[irange] int [-32, -31] NONZERO 0xffffffe0

The maybe nonzero bits are wrong as -31 has the LSB set (0xffffffe1)
whereas the bitmask says the lower 4 bits are off.

	PR tree-optimization/107541

gcc/ChangeLog:

	* range-op.cc (operator_div::fold_range): Restrict power of 2
	optimization to positive numbers.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/pr107541.c: New test.