aco/jump_threading: don't remove loop preheaders

They might be needed as convergence point in order to
insert code (e.g. for loop alignment, wait states, etc.).

Totals from 1 (0.00% of 79395) affected shaders:

CodeSize: 12672 -> 12716 (+0.35%)
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32477>

src/amd/compiler/aco_jump_threading.cpp

@@ -109,6 +109,12 @@ try_remove_simple_block(jump_threading_ctx& ctx, Block* block)
    if (!is_empty_block(block, false))
       return;
 
+   /* Don't remove the preheader as it might be needed as convergence point
+    * in order to insert code (e.g. for loop alignment, wait states, etc.).
+    */
+   if (block->kind & block_kind_loop_preheader)
+      return;
+
    Block& pred = ctx.program->blocks[block->linear_preds[0]];
    Block& succ = ctx.program->blocks[block->linear_succs[0]];
    Pseudo_branch_instruction& branch = pred.instructions.back()->branch();