[csa] Assert we start with depth != 0 in GotoIfHasContextExtensionUpToDepth

Since we'll immediately run the first check.

Bug: v8:11429
Change-Id: I47de46d8281b42c90d5cd1e1cbbfc6232c752da1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2700672
Auto-Submit: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72846}

src/codegen/code-stub-assembler.cc

@@ -11056,7 +11056,7 @@ TNode<Context> CodeStubAssembler::GotoIfHasContextExtensionUpToDepth(
   Label no_extension(this);
 
   // Loop until the depth is 0.
-  // TODO(v8:11429): Assert that cur_depth isn't zero to start with.
+  CSA_ASSERT(this, Word32NotEqual(cur_depth.value(), Int32Constant(0)));
   Goto(&context_search);
   BIND(&context_search);
   {