[codegen] add new recipe "rout" (#1014)

* [codegen] add new recipe "rout"

Add a new recipe "rout" intended to be used by arithematic operations
that output flags, currently being used for `iadd_cout` and `isub_bout`.

Fixes: https://github.com/CraneStation/cranelift/issues/1009

cranelift/filetests/filetests/isa/x86/run-i64.clif

@@ -0,0 +1,17 @@
+; Test i64 instructions on x86_32.
+test compile
+target i686 haswell
+
+function %iadd(i64, i64) -> i64 {
+ebb0(v1: i64, v2: i64):
+    v10 = iadd v1, v2
+    ; check: iadd_cout
+    return v10
+}
+
+function %isub(i64, i64) -> i64 {
+ebb0(v1: i64, v2: i64):
+    v10 = isub v1, v2
+    ; check: isub_bout
+    return v10
+}