nir: Introduce a nir_opt_move_comparisons() pass.

This tries to move comparisons (a common source of boolean values)
closer to their first use.  For GPUs which use condition codes,
this can eliminate a lot of temporary booleans and comparisons
which reload the condition code register based on a boolean.

V2: (Timothy Arceri)
 - fix move comparision for phis so we dont end up with:

    vec1 32 ssa_227 = phi block_34: ssa_1, block_38: ssa_240
    vec1 32 ssa_235 = feq ssa_227, ssa_1
    vec1 32 ssa_230 = phi block_34: ssa_221, block_38: ssa_235

 - add nir_op_i2b/nir_op_f2b to the list of comparisons.

V3: (Timothy Arceri)
 - tidy up suggested by Jason.
 - add inot/fnot to move comparison list

V4: (Jason Ekstrand)
 - clean up move_comparison_source
 - get rid of the tuple
 - rework phi handling

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Ian Romanick <[email protected]> [v1]
Reviewed-by: Jason Ekstrand <[email protected]>

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