agx: Make partial DCE optional

Our dead code elimination pass does two things:

1. delete instructions that are entirely unnecessary
2. delete unnecessary destinations of necessary instructions

To deal with pass ordering issues, we sometimes want to do #1 without #2.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21674>

src/asahi/compiler/test/test-optimizer.cpp

@@ -29,7 +29,7 @@ static void
 agx_optimize_and_dce(agx_context *ctx)
 {
    agx_optimizer(ctx);
-   agx_dce(ctx);
+   agx_dce(ctx, true);
 }
 
 #define CASE(instr, expected, size)                                            \