nir: Add partial redundancy elimination for compares

nir: Add partial redundancy elimination for compares

This pass attempts to dectect code sequences like

    if (x < y) {
        z = y - x;
	...
    }

and replace them with sequences like

    t = x - y;
    if (t < 0) {
        z = -t;
	...
    }

On architectures where the subtract can generate the flags used by the
if-statement, this saves an instruction.  It's also possible that moving
an instruction out of the if-statement will allow
nir_opt_peephole_select to convert the whole thing to a bcsel.

Currently only floating point compares and adds are supported.  Adding
support for integer will be a challenge due to integer overflow.  There
are a couple possible solutions, but they may not apply to all
architectures.

v2: Fix a typo in the commit message and a couple typos in comments.
Fix possible NULL pointer deref from result of push_block().  Add
missing (-A + B) case.  Suggested by Caio.

v3: Fix is_not_const_zero to work correctly with types other than
nir_type_float32.  Suggested by Ken.

v4: Add some comments explaining how this works.  Suggested by Ken.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>