Handle ABI arguments correctly in the reload pass.

Values passed as arguments to calls and return instructions may also be
reload candidates.

filetests/regalloc/spill.cton

@@ -14,10 +14,15 @@ test regalloc
 isa riscv enable_e
 
 ; In straight-line code, the first value defined is spilled.
-; That is the argument.
+; That is in order:
+; 1. The argument v1.
+; 2. The link register.
 function %pyramid(i32) -> i32 {
 ebb0(v1: i32):
-; check: $v1 = spill $(rv1=$V)
+; check: $ebb0($(rv1=$V): i32, $(rlink=$V): i32)
+    ; check: $v1 = spill $rv1
+    ; nextln: $(link=$V) = spill $rlink
+    ; not: spill
     v2 = iadd_imm v1, 12
     v3 = iadd_imm v2, 12
     v4 = iadd_imm v3, 12
@@ -29,7 +34,9 @@ ebb0(v1: i32):
     v10 = iadd_imm v9, 12
     v11 = iadd_imm v10, 12
     v12 = iadd_imm v11, 12
-    v31 = iadd v11, v12
+    v13 = iadd_imm v12, 12
+    v32 = iadd v12, v13
+    v31 = iadd v32, v11
     v30 = iadd v31, v10
     v29 = iadd v30, v9
     v28 = iadd v29, v8
@@ -40,5 +47,7 @@ ebb0(v1: i32):
     v23 = iadd v24, v3
     v22 = iadd v23, v2
     v21 = iadd v22, v1
+    ; check: $(rlink2=$V) = fill $link
     return v21
+    ; check: return $v21, $rlink2
 }