Fix DenseMapInfo<AliasKeyTy>::getEmptyKey

The all zero key is not a good empty key because it is a legal value.

The all zero key means that we are comparing some value to itself (index zero in
the value table), looking at the first SILValue value with no type information
(for nullptr type info).

Author: nadavrot

include/swift/SILAnalysis/AliasAnalysis.h

@@ -236,7 +236,8 @@ bool isLetPointer(SILValue V);
 namespace llvm {
   template <> struct llvm::DenseMapInfo<AliasKeyTy> {
     static inline AliasKeyTy getEmptyKey() {
-      return {0, 0, 0, 0, nullptr, nullptr};
+      auto Allone = std::numeric_limits<size_t>::max();
+      return {Allone, Allone, 0xffff, 0xffff, nullptr, nullptr};
     }
     static inline AliasKeyTy getTombstoneKey() {
       auto Allone = std::numeric_limits<size_t>::max();