intel/compiler: Silence warning about value that may be used uninitialized

intel/compiler: Silence warning about value that may be used uninitialized

For some reason, this warning only occurs for me in release builds.

In file included from src/intel/compiler/brw_nir_lower_mem_access_bit_sizes.c:25:0:
src/intel/compiler/brw_nir_lower_mem_access_bit_sizes.c: In function ‘brw_nir_lower_mem_access_bit_sizes’:
src/compiler/nir/nir_builder.h:501:26: warning: ‘src_swiz[2]’ may be used uninitialized in this function [-Wmaybe-uninitialized]
       alu_src.swizzle[i] = swiz[i];
       ~~~~~~~~~~~~~~~~~~~^~~~~~~~~
src/intel/compiler/brw_nir_lower_mem_access_bit_sizes.c:225:16: note: ‘src_swiz[2]’ was declared here
       unsigned src_swiz[4];
                ^~~~~~~~

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>