nir/opt_if: Remove unneeded phis if we make progress

Now that SSA values can be derefs and they have special rules, we have
to be a bit more careful about our LCSSA phis.  In particular, we need
to clean up in case LCSSA ended up creating a phi node for a deref.
This fixes validation issues with some Vulkan CTS tests with the new
deref instructions.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>

src/compiler/nir/nir.h

@@ -2903,6 +2903,7 @@ bool nir_opt_move_load_ubo(nir_shader *shader);
 
 bool nir_opt_peephole_select(nir_shader *shader, unsigned limit);
 
+bool nir_opt_remove_phis_impl(nir_function_impl *impl);
 bool nir_opt_remove_phis(nir_shader *shader);
 
 bool nir_opt_shrink_load(nir_shader *shader);

src/compiler/nir/nir_opt_if.c

@@ -403,6 +403,13 @@ nir_opt_if(nir_shader *shader)
           * that don't dominate their uses.
           */
          nir_lower_regs_to_ssa_impl(function->impl);
+
+         /* Calling nir_convert_loop_to_lcssa() in opt_peel_loop_initial_if()
+          * adds extra phi nodes which may not be valid if they're used for
+          * something such as a deref.  Remove any unneeded phis.
+          */
+         nir_opt_remove_phis_impl(function->impl);
+
          progress = true;
       }
    }

src/compiler/nir/nir_opt_remove_phis.c

@@ -139,8 +139,8 @@ remove_phis_block(nir_block *block, nir_builder *b)
    return progress;
 }
 
-static bool
-remove_phis_impl(nir_function_impl *impl)
+bool
+nir_opt_remove_phis_impl(nir_function_impl *impl)
 {
    bool progress = false;
    nir_builder bld;
@@ -165,7 +165,7 @@ nir_opt_remove_phis(nir_shader *shader)
 
    nir_foreach_function(function, shader)
       if (function->impl)
-         progress = remove_phis_impl(function->impl) || progress;
+         progress = nir_opt_remove_phis_impl(function->impl) || progress;
 
    return progress;
 }