Add fixed constraints for ABI arguments and return values.

We can start adding some real test cases for the move resolver now.

filetests/regalloc/basic.cton

@@ -3,6 +3,8 @@ test regalloc
 ; We can add more ISAs once they have defined encodings.
 isa riscv
 
+; regex: RX=%x\d+
+
 function add(i32, i32) {
 ebb0(v1: i32, v2: i32):
     v3 = iadd v1, v2
@@ -14,6 +16,27 @@ ebb0(v1: i32, v2: i32):
 ; Function with a dead argument.
 function dead_arg(i32, i32) -> i32{
 ebb0(v1: i32, v2: i32):
+; not: regmove
 ; check: return $v1
     return v1
 }
+
+; Return a value from a different register.
+function move1(i32, i32) -> i32 {
+ebb0(v1: i32, v2: i32):
+; not: regmove
+; check: regmove $v2, %x11 -> %x10
+; nextln: return $v2
+        return v2
+}
+
+; Swap two registers.
+function swap(i32, i32) -> i32, i32 {
+ebb0(v1: i32, v2: i32):
+; not: regmove
+; check: regmove $v2, %x11 -> $(tmp=$RX)
+; nextln: regmove $v1, %x10 -> %x11
+; nextln: regmove $v2, $tmp -> %x10
+; nextln: return $v2, $v1
+        return v2, v1
+}