nir: mark *prev_block as MAYBE_UNUSED in opt_peel_loop_initial_if

Only used, when asserts are enabled.

Fixes an unused-variable warning with gcc-8:
 ../../../src/compiler/nir/nir_opt_if.c: In function 'opt_peel_loop_initial_if':
 ../../../src/compiler/nir/nir_opt_if.c:109:15: warning: unused variable 'prev_block' [-Wunused-variable]
     nir_block *prev_block =
                ^~~~~~~~~~

Signed-off-by: Kai Wasserbäch <kai@dev.carbon-project.org>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/compiler/nir/nir_opt_if.c

@@ -106,7 +106,7 @@ static bool
 opt_peel_loop_initial_if(nir_loop *loop)
 {
    nir_block *header_block = nir_loop_first_block(loop);
-   nir_block *prev_block =
+   MAYBE_UNUSED nir_block *prev_block =
       nir_cf_node_as_block(nir_cf_node_prev(&loop->cf_node));
 
    /* It would be insane if this were not true */