match.pd: Optimize (x < 0) ^ (y < 0) to (x ^ y) < 0 etc. [PR96681]

This patch simplifies comparisons that test the sign bit xored together.
If the comparisons are both < 0 or both >= 0, then we should xor the operands
together and compare the result to < 0, if the comparisons are different,
we should compare to >= 0.

2021-01-15  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/96681
	* match.pd ((x < 0) ^ (y < 0) to (x ^ y) < 0): New simplification.
	((x >= 0) ^ (y >= 0) to (x ^ y) < 0): Likewise.
	((x < 0) ^ (y >= 0) to (x ^ y) >= 0): Likewise.
	((x >= 0) ^ (y < 0) to (x ^ y) >= 0): Likewise.

	* gcc.dg/tree-ssa/pr96681.c: New test.