Add helper routines for special-purpose arguments.

- ArgumentType::special() creates a new special-purpose argument without
  assigning it to a register location.
- Signature::special_arg_index() funds a unique special-purpose
  argument.
- Function::special_arg() finds a special-purpose argument by value.

Also add a new "sigid" argument purpose which will be used for runtime
signature checks in WebAssembly indirect calls.

lib/cretonne/src/legalizer/boundary.rs

@@ -57,6 +57,7 @@ fn legalize_entry_arguments(func: &mut Function, entry: Ebb) {
     let mut has_sret = false;
     let mut has_link = false;
     let mut has_vmctx = false;
+    let mut has_sigid = false;
 
     // Insert position for argument conversion code.
     // We want to insert instructions before the first instruction in the entry block.
@@ -91,6 +92,10 @@ fn legalize_entry_arguments(func: &mut Function, entry: Ebb) {
                     assert!(!has_vmctx, "Multiple vmctx arguments found");
                     has_vmctx = true;
                 }
+                ArgumentPurpose::SignatureId => {
+                    assert!(!has_sigid, "Multiple sigid arguments found");
+                    has_sigid = true;
+                }
                 _ => panic!("Unexpected special-purpose arg {}", abi_types[abi_arg]),
             }
             abi_arg += 1;
@@ -145,6 +150,10 @@ fn legalize_entry_arguments(func: &mut Function, entry: Ebb) {
                 assert!(!has_vmctx, "Multiple vmctx arguments found");
                 has_vmctx = true;
             }
+            ArgumentPurpose::SignatureId => {
+                assert!(!has_sigid, "Multiple sigid arguments found");
+                has_sigid = true;
+            }
         }
 
         // Just create entry block values to match here. We will use them in `handle_return_abi()`