simplify comparisons followed by an equality test against 0 or 1

simplify comparisons followed by an equality test against 0 or 1

Expressions involving equality testing against zero are ubiquitious
and can often be simplified with previous comparisons.

For example, when using test-linearize on the following code:
	_Bool foo(int a) { return !(a < 3); }
the following was emitted:
	setlt.32    %r2 <- %arg1, $3
	seteq.32    %r3 <- %r2, $0
	setne.1     %r4 <- %r3, $0
	ret.1       %r4

but this can be simplified into:
	setge.1     %r4 <- %arg1, $3
	ret.1       %r4

Implement this simplification and add associated test cases.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Christopher Li <sparse@chrisli.org>