Print constants in a comment in CLIF output (#4725)

When trying to read generated CLIF, it's nice to be able to see at a
glance that some of the operands are defined by `iconst` and similar
instructions, without having to go find each operand's definition
manually.

cranelift/tests/bugpoint_test_expected.clif

@@ -30,6 +30,6 @@ block0:
     v990 -> v1052
     v1051 -> v1052
     v1055 -> v1052
-    call fn0(v0, v105, v1052, v883, v829, v987, v951, v842)
+    call fn0(v0, v105, v1052, v883, v829, v987, v951, v842)  ; v0 = 0, v105 = 0, v1052 = 0, v883 = 0, v829 = 0, v987 = 0, v951 = 0, v842 = 0
     trap user0
 }