nir: evaluate if condition uses inside the if branches

Since we know what side of the branch we ended up on we can just
replace the use with a constant.

All the spill changes in shader-db are from Dolphin uber shaders,
despite some small regressions the change is clearly positive.

V2: insert new constant after any phis in the
    use->parent_instr->type == nir_instr_type_phi path.

v3:
 - use nir_after_block_before_jump() for inserting const
 - check dominance of phi uses correctly

v4:
 - create some helpers as suggested by Jason.

v5 (Jason Ekstrand):
 - Use LIST_ENTRY to get the phi src

shader-db results IVB:

total instructions in shared programs: 9999201 -> 9993483 (-0.06%)
instructions in affected programs: 163235 -> 157517 (-3.50%)
helped: 132
HURT: 2

total cycles in shared programs: 231670754 -> 219476091 (-5.26%)
cycles in affected programs: 143424120 -> 131229457 (-8.50%)
helped: 115
HURT: 24

total spills in shared programs: 4383 -> 4370 (-0.30%)
spills in affected programs: 1656 -> 1643 (-0.79%)
helped: 9
HURT: 18

total fills in shared programs: 4610 -> 4581 (-0.63%)
fills in affected programs: 374 -> 345 (-7.75%)
helped: 6
HURT: 0

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/nir/nir.h

@@ -2353,6 +2353,36 @@ nir_after_block_before_jump(nir_block *block)
    }
 }
 
+static inline nir_cursor
+nir_before_src(nir_src *src, bool is_if_condition)
+{
+   if (is_if_condition) {
+      nir_block *prev_block =
+         nir_cf_node_as_block(nir_cf_node_prev(&src->parent_if->cf_node));
+      assert(!nir_block_ends_in_jump(prev_block));
+      return nir_after_block(prev_block);
+   } else if (src->parent_instr->type == nir_instr_type_phi) {
+#ifndef NDEBUG
+      nir_phi_instr *cond_phi = nir_instr_as_phi(src->parent_instr);
+      bool found = false;
+      nir_foreach_phi_src(phi_src, cond_phi) {
+         if (phi_src->src.ssa == src->ssa) {
+            found = true;
+            break;
+         }
+      }
+      assert(found);
+#endif
+      /* The LIST_ENTRY macro is a generic container-of macro, it just happens
+       * to have a more specific name.
+       */
+      nir_phi_src *phi_src = LIST_ENTRY(nir_phi_src, src, src);
+      return nir_after_block_before_jump(phi_src->pred);
+   } else {
+      return nir_before_instr(src->parent_instr);
+   }
+}
+
 static inline nir_cursor
 nir_before_cf_node(nir_cf_node *node)
 {