Now diagnosing missing vmctx arguments (fixes #376) (#384)

* Now diagnosing missing vmctx arguments (fixes #376).

* Added filetest for fix of #376.

* Respect formatting rules in verifier/mod.rs.

* Added parameters for each use of vmctx in test files.

* Added comments on additions on vmctx verifications.

cranelift/filetests/verifier/undeclared_vmctx.cton

@@ -0,0 +1,17 @@
+test verifier
+
+; Using a vmctx global value without declaring it first leads to an error.
+function %vmglobal_err(i64) -> i64 {
+    gv4 = vmctx+0 ; error: undeclared vmctx reference
+ebb0(v0: i64):
+    v1 = global_value.i64 gv4
+    return v1
+}
+
+; If it is declared, all is fine.
+function %vmglobal_ok(i64 vmctx) -> i64 {
+    gv4 = vmctx+0
+ebb0(v0: i64):
+    v1 = global_value.i64 gv4
+    return v1
+}