Fix verifier bug in unreachable code.

We want to disable dominance checks in unreachable code. The
is_reachable() check for EBB parameter values was checking if the
defining EBB was reachable, not the EBB using the value.

This bug showed up in fuzzing and in #213.

cranelift/filetests/verifier/unreachable_code.cton

@@ -21,3 +21,25 @@ ebb9(v7: i32):
     return v9
 
 }
+
+; Using a function argument in an unreachable block is ok.
+function %arg(i32) -> i32 {
+ebb0(v0: i32):
+    v1 = iadd_imm v0, 1
+    return v1
+
+ebb1:
+    v10 = iadd_imm v0, 10
+    return v10
+}
+
+; Using an EBB argument from an unreachable block is not ok.
+function %arg2(i32) -> i32 {
+ebb0(v0: i32):
+    v1 = iadd v0, v10   ; error: uses value arg from non-dominating
+    return v1
+
+ebb1(v10: i32):
+    v11 = iadd v0, v10
+    return v11
+}