Insert copies to support a value being used as multiple return values.

When one value is used multiple times for separate return values, we
need to copy it to produce a new value, so that each value can be
allocated a different register.

cranelift/filetests/regalloc/multiple-returns.clif

@@ -0,0 +1,13 @@
+test regalloc
+target x86_64
+
+; Return the same value twice. This needs a copy so that each value can be
+; allocated its own register.
+function %multiple_returns() -> i64, i64 {
+ebb0:
+    v2 = iconst.i64 0
+    return v2, v2
+}
+; check: v2 = iconst.i64 0
+; check: v3 = copy v2
+; check: return v2, v3