[sil] Add a small helper API called SILUndef::get(SILValue).

This just creates a new SILUndef with the same type as the SILValue. This just
prevents one from having to write
SILUndef::get(value->getType(), *value->getModule()).

(cherry picked from commit 92116ba)

Author: gottesmm

include/swift/SIL/SILUndef.h

@@ -30,6 +30,12 @@ class SILUndef : public ValueBase {
   void operator delete(void *, size_t) = delete;
 
   static SILUndef *get(SILType ty, SILModule &m);
+
+  /// Return a SILUndef with the same type as the passed in value.
+  static SILUndef *get(SILValue value) {
+    return SILUndef::get(value->getType(), *value->getModule());
+  }
+
   static SILUndef *get(SILType ty, const SILFunction &f);
 
   template <class OwnerTy>