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>