Constant-fold icmp instructions (#5666)

We found examples of icmp instructions with both operands constant in
spidermonkey.wasm.

cranelift/codegen/src/opts/cprop.isle

@@ -70,6 +70,14 @@
                       (iconst ty k2)))
       (subsume (iconst ty (imm64_sshr ty k1 k2))))
 
+(rule (simplify
+       (icmp result_ty
+            cc
+            (iconst ty k1)
+            (iconst ty k2)))
+      (subsume (iconst result_ty (imm64_icmp ty cc k1 k2))))
+
+
 ;; Canonicalize via commutativity: push immediates to the right.
 ;;
 ;;   (op k x) --> (op x k)