middle-end: simplify complex if expressions where comparisons are inverse of one another.

This optimizes the following sequence

  ((a < b) & c) | ((a >= b) & d)

into

  (a < b ? c : d) & 1

for scalar and on vector we can omit the & 1.

Also recognizes

  (-(a < b) & c) | (-(a >= b) & d)

into

  a < b ? c : d

This changes the code generation from

zoo2:
	cmp     w0, w1
	cset    w0, lt
	cset    w1, ge
	and     w0, w0, w2
	and     w1, w1, w3
	orr     w0, w0, w1
	ret

into

	cmp	w0, w1
	csel	w0, w2, w3, lt
	and	w0, w0, 1
	ret

and significantly reduces the number of selects we have to do in the vector
code.

gcc/ChangeLog:

	* match.pd: Add new rule.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/if-compare_1.c: New test.
	* gcc.target/aarch64/if-compare_2.c: New test.