aco/optimizer: ensure to not erase high bits when propagating packed constants

Packed constants with non-zero values in the high half
might have been propagated as 16 bit, dropping the high half.

Cc: mesa-stable
Closes: #5070
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11954>

src/amd/compiler/aco_optimizer.cpp

@@ -194,7 +194,8 @@ struct ssa_info {
       add_label(label_literal);
       val = constant;
 
-      if (chip >= GFX8 && !op16.isLiteral())
+      /* check that no upper bits are lost in case of packed 16bit constants */
+      if (chip >= GFX8 && !op16.isLiteral() && op16.constantValue64() == constant)
          add_label(label_constant_16bit);
 
       if (!op32.isLiteral())