[Test] Add regression test.

For #70234 .

Author: nate-chandler

validation-test/SILOptimizer/gh70234.swift

@@ -0,0 +1,12 @@
+// RUN: %target-swift-frontend -emit-sil -verify %s
+
+// REQUIRES: objc_interop
+
+import Foundation
+
+func mutateText(_ rawText: NSString?) -> String {
+  guard let text = consume rawText else {
+    return "text unavailable"
+  }
+  return String(text)
+}