Append link and sret arguments in legalize_signature.

These special-purpose arguments and return values are only relevant for
the function being compiled, so add a `current` flag to
legalize_signature().

- Add the necessary argument values to the entry block to represent
  the special-purpose arguments.
- Propagate the link and sret arguments to return instructions if the
  legalized signature asks for it.

filetests/isa/riscv/split-args.cton

@@ -6,7 +6,7 @@ isa riscv
 
 function simple(i64, i64) -> i64 {
 ebb0(v1: i64, v2: i64):
-; check: $ebb0($(v1l=$V): i32, $(v1h=$V): i32, $(v2l=$V): i32, $(v2h=$V): i32):
+; check: $ebb0($(v1l=$V): i32, $(v1h=$V): i32, $(v2l=$V): i32, $(v2h=$V): i32, $(link=$V): i32):
     jump ebb1(v1)
     ; check: jump $ebb1($v1l, $v1h)
 
@@ -16,12 +16,12 @@ ebb1(v3: i64):
     ; check: $(v4l=$V) = band $v3l, $v2l
     ; check: $(v4h=$V) = band $v3h, $v2h
     return v4
-    ; check: return $v4l, $v4h
+    ; check: return $v4l, $v4h, $link
 }
 
 function multi(i64) -> i64 {
 ebb1(v1: i64):
-; check: $ebb1($(v1l=$V): i32, $(v1h=$V): i32):
+; check: $ebb1($(v1l=$V): i32, $(v1h=$V): i32, $(link=$V): i32):
     jump ebb2(v1, v1)
     ; check: jump $ebb2($v1l, $v1l, $v1h, $v1h)
 
@@ -36,12 +36,12 @@ ebb3(v4: i64):
     ; check: $(v5l=$V) = band $v4l, $v3l
     ; check: $(v5h=$V) = band $v4h, $v3h
     return v5
-    ; check: return $v5l, $v5h
+    ; check: return $v5l, $v5h, $link
 }
 
 function loop(i64, i64) -> i64 {
 ebb0(v1: i64, v2: i64):
-; check: $ebb0($(v1l=$V): i32, $(v1h=$V): i32, $(v2l=$V): i32, $(v2h=$V): i32):
+; check: $ebb0($(v1l=$V): i32, $(v1h=$V): i32, $(v2l=$V): i32, $(v2h=$V): i32, $(link=$V): i32):
     jump ebb1(v1)
     ; check: jump $ebb1($v1l, $v1h)