aco: Fix maybe-uninitialized warnings.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3483>

src/amd/compiler/aco_assembler.cpp

@@ -512,6 +512,8 @@ void emit_instruction(asm_context& ctx, std::vector<uint32_t>& out, Instruction*
             encoding = (0b110100 << 26);
          } else if (ctx.chip_class == GFX10) {
             encoding = (0b110101 << 26);
+         } else {
+            unreachable("Unknown chip_class.");
          }
 
          if (ctx.chip_class <= GFX7) {

src/amd/compiler/aco_instruction_selection.cpp

@@ -7937,6 +7937,8 @@ static void emit_stream_output(isel_context *ctx,
       case 4:
          opcode = aco_opcode::buffer_store_dwordx4;
          break;
+      default:
+         unreachable("Unsupported dword count.");
       }
 
       aco_ptr<MUBUF_instruction> store{create_instruction<MUBUF_instruction>(opcode, Format::MUBUF, 4, 0)};