agx: Fix float copyprop of neg(neg) case

They should cancel each other. Fixes:

   dEQP-GLES2.functional.shaders.random.basic_expression.combined.0

Fixes: 28801b4849 ("agx: Add forward optimizing pass for fmov")
Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18380>

src/asahi/compiler/agx_optimizer.c

@@ -81,11 +81,12 @@ agx_is_fmov(agx_instr *def)
 static agx_index
 agx_compose_float_src(agx_index to, agx_index from)
 {
-   if (to.abs)
+   if (to.abs) {
       from.neg = false;
+      from.abs = true;
+   }
 
-   from.abs |= to.abs;
-   from.neg |= to.neg;
+   from.neg ^= to.neg;
 
    return from;
 }