Add operator bool for MemoryLocations.

[bryant]

Summary

The primary benefit of this patch is being able to write stuff like:

// getLocForWrite returns `MemoryLocation()` upon failure.
if (MemoryLocation M = getLocForWrite(Instruction, AA)) {
    // do stuff with M
}

If there are any down sides, please let me know.

[efriedma]

I'm not sure we want to encourage the use of null MemoryLocations... maybe getLocForWrite() should use Optional<MemoryLocation> instead?

include/llvm/Analysis/MemoryLocation.h

122

operator bool() should always be explicit.

include/llvm/Analysis/MemoryLocation.h

MemoryLocation getWithoutAATags() const {
     MemoryLocation Copy(*this);
     Copy.AATags = AAMDNodes();
     return Copy;
   }
 
   bool operator==(const MemoryLocation &Other) const {
     return Ptr == Other.Ptr && Size == Other.Size && AATags == Other.AATags;
   }
+
+  operator bool() const { return Ptr; }
 };
 
 // Specialize DenseMapInfo for MemoryLocation.
 template <> struct DenseMapInfo<MemoryLocation> {
   static inline MemoryLocation getEmptyKey() {
     return MemoryLocation(DenseMapInfo<const Value *>::getEmptyKey(), 0);
   }
   static inline MemoryLocation getTombstoneKey() {