Stack Limit as an Argument Purpose (#372)

* Initial approach.

* Move stack_limit check before opening the frame

* Account for GPRs and frame pointer in stack check

* Check stack_limit example.

* Remove stack_limit attribute code.

Amends #359

* fmt

lib/codegen/src/legalizer/boundary.rs

@@ -73,6 +73,7 @@ fn legalize_entry_params(func: &mut Function, entry: Ebb) {
     let mut has_link = false;
     let mut has_vmctx = false;
     let mut has_sigid = false;
+    let mut has_stack_limit = false;
 
     // Insert position for argument conversion code.
     // We want to insert instructions before the first instruction in the entry block.
@@ -111,6 +112,10 @@ fn legalize_entry_params(func: &mut Function, entry: Ebb) {
                     debug_assert!(!has_sigid, "Multiple sigid arguments found");
                     has_sigid = true;
                 }
+                ArgumentPurpose::StackLimit => {
+                    debug_assert!(!has_stack_limit, "Multiple stack_limit arguments found");
+                    has_stack_limit = true;
+                }
                 _ => panic!("Unexpected special-purpose arg {}", abi_type),
             }
             abi_arg += 1;
@@ -167,6 +172,10 @@ fn legalize_entry_params(func: &mut Function, entry: Ebb) {
                 debug_assert!(!has_sigid, "Multiple sigid parameters found");
                 has_sigid = true;
             }
+            ArgumentPurpose::StackLimit => {
+                debug_assert!(!has_stack_limit, "Multiple stack_limit parameters found");
+                has_stack_limit = true;
+            }
         }
 
         // Just create entry block values to match here. We will use them in `handle_return_abi()`