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/legalize-i64.cton

@@ -9,39 +9,39 @@ ebb0(v1: i64, v2: i64):
     v3 = band v1, v2
     return v3
 }
-; 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):
 ; check: [R#ec
 ; sameln: $(v3l=$V) = band $v1l, $v2l
 ; check: [R#ec
 ; sameln: $(v3h=$V) = band $v1h, $v2h
 ; check: $v3 = iconcat $v3l, $v3h
-; check: return $v3l, $v3h
+; check: return $v3l, $v3h, $link
 
 function bitwise_or(i64, i64) -> i64 {
 ebb0(v1: i64, v2: i64):
     v3 = bor v1, v2
     return v3
 }
-; 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):
 ; check: [R#cc
 ; sameln: $(v3l=$V) = bor $v1l, $v2l
 ; check: [R#cc
 ; sameln: $(v3h=$V) = bor $v1h, $v2h
 ; check: $v3 = iconcat $v3l, $v3h
-; check: return $v3l, $v3h
+; check: return $v3l, $v3h, $link
 
 function bitwise_xor(i64, i64) -> i64 {
 ebb0(v1: i64, v2: i64):
     v3 = bxor v1, v2
     return v3
 }
-; 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):
 ; check: [R#8c
 ; sameln: $(v3l=$V) = bxor $v1l, $v2l
 ; check: [R#8c
 ; sameln: $(v3h=$V) = bxor $v1h, $v2h
 ; check: $v3 = iconcat $v3l, $v3h
-; check: return $v3l, $v3h
+; check: return $v3l, $v3h, $link
 
 function arith_add(i64, i64) -> i64 {
 ; Legalizing iadd.i64 requires two steps:
@@ -51,7 +51,7 @@ ebb0(v1: i64, v2: i64):
     v3 = iadd v1, v2
     return v3
 }
-; 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):
 ; check: [R#0c
 ; sameln: $(v3l=$V) = iadd $v1l, $v2l
 ; check: $(c=$V) = icmp ult $v3l, $v1l
@@ -61,4 +61,4 @@ ebb0(v1: i64, v2: i64):
 ; check: [R#0c
 ; sameln: $(v3h=$V) = iadd $v3h1, $c_int
 ; check: $v3 = iconcat $v3l, $v3h
-; check: return $v3l, $v3h
+; check: return $v3l, $v3h, $link