Add a test with a fixed register constraint.

Make sure we use the diverted register location for tied operands.

filetests/regalloc/constraints.cton

@@ -13,3 +13,16 @@ ebb0:
     v2 = isub v0, v1
     return v2
 }
+
+; Fixed register constraint.
+function %fixed_op() -> i32 {
+ebb0:
+    ; check: ,%rax]
+    ; sameln: $v0 = iconst.i32 12
+    v0 = iconst.i32 12
+    v1 = iconst.i32 13
+    ; The dynamic shift amount must be in %rcx
+    ; check: regmove $v0, %rax -> %rcx
+    v2 = ishl v1, v0
+    return v2
+}