aco: Fix signedness compare warning.

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

src/amd/compiler/aco_insert_exec_mask.cpp

@@ -732,7 +732,7 @@ void process_instructions(exec_ctx& ctx, Block* block,
          int num;
          Temp cond, exit_cond;
          if (instr->operands[0].isConstant()) {
-            assert(instr->operands[0].constantValue() == -1);
+            assert(instr->operands[0].constantValue() == -1u);
             /* transition to exact and set exec to zero */
             Temp old_exec = ctx.info[block->index].exec.back().first;
             Temp new_exec = bld.tmp(bld.lm);