vc4: Optimize out a comparison for bcsel based on an ALU comparison

vc4: Optimize out a comparison for bcsel based on an ALU comparison

We routinely have code like:

	vec1 ssa_220 = fge ssa_104, ssa_61
	vec1 ssa_199 = bcsel ssa_220, ssa_106, ssa_105

and we would compare fge's args and choose between ~0 and 0 to generate
ssa_220, then compare ssa_220 to 0 and choose between bcsel's args.
Instead, try to notice the pattern and compare between fge's args to
select between bcsel's args.

total instructions in shared programs: 88019 -> 87574 (-0.51%)
instructions in affected programs:     9985 -> 9540 (-4.46%)
total estimated cycles in shared programs: 245752 -> 245237 (-0.21%)
estimated cycles in affected programs:     17232 -> 16717 (-2.99%)