nir/remove_phis: handle trivial back-edges

nir/remove_phis: handle trivial back-edges

Some loops may have phi nodes that look like:

foo = ...
loop {
    bar = phi(foo, bar)
    ...
}

in which case we can remove the phi node and replace all uses of 'bar'
with 'foo'. In particular, there are some L4D2 vertex shaders with loops
that, after optimization, look like:

        /* succs: block_1 */
        loop {
                block block_1:
                /* preds: block_0 block_4 */
                vec1 ssa_2195 = phi block_0: ssa_2136, block_4: ssa_994
                vec1 ssa_7321 = phi block_0: ssa_8195, block_4: ssa_7321
                vec1 ssa_7324 = phi block_0: ssa_8198, block_4: ssa_7324
                vec1 ssa_7327 = phi block_0: ssa_8174, block_4: ssa_7327
                vec1 ssa_8139 = intrinsic load_uniform () () (232)
                vec1 ssa_588 = ige ssa_2195, ssa_8139
                /* succs: block_2 block_3 */
                if ssa_588 {
                        block block_2:
                        /* preds: block_1 */
                        break
                        /* succs: block_5 */
                } else {
                        block block_3:
                        /* preds: block_1 */
                        /* succs: block_4 */
                }
                block block_4:
                /* preds: block_3 */
                vec1 ssa_994 = iadd ssa_2195, ssa_2150
                /* succs: block_1 */
        }

where after removing the second, third, and fourth phi nodes, the loop becomes
entirely dead, and this patch will cause the loop to be deleted entirely.

No piglit regressions.

Shader-db results on bdw:

instructions in affected programs:     5824 -> 5664 (-2.75%)
total loops in shared programs:        2234 -> 2202 (-1.43%)
helped:                                32

Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Connor Abbott <cwabbott0@gmail.com>