[ic] Remove pointless xor from primary StubCache hash computation

This xor can never change the number of collisions, so it should be safe to remove.

Bug: 
Change-Id: I253c0ece422f66e7cba15b13c041cfb6c8361674
Reviewed-on: https://chromium-review.googlesource.com/809113Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49985}

src/ic/accessor-assembler.cc

@@ -2076,9 +2076,8 @@ Node* AccessorAssembler::StubCachePrimaryOffset(Node* name, Node* map) {
   // risk of collision even if the heap is spread over an area larger than
   // 4Gb (and not at all if it isn't).
   Node* map32 = TruncateWordToWord32(BitcastTaggedToWord(map));
-  Node* hash = Int32Add(hash_field, map32);
   // Base the offset on a simple combination of name and map.
-  hash = Word32Xor(hash, Int32Constant(StubCache::kPrimaryMagic));
+  Node* hash = Int32Add(hash_field, map32);
   uint32_t mask = (StubCache::kPrimaryTableSize - 1)
                   << StubCache::kCacheIndexShift;
   return ChangeUint32ToWord(Word32And(hash, Int32Constant(mask)));

src/ic/stub-cache.cc

@@ -39,7 +39,7 @@ int StubCache::PrimaryOffset(Name* name, Map* map) {
   uint32_t map_low32bits =
       static_cast<uint32_t>(reinterpret_cast<uintptr_t>(map));
   // Base the offset on a simple combination of name and map.
-  uint32_t key = (map_low32bits + field) ^ kPrimaryMagic;
+  uint32_t key = map_low32bits + field;
   return key & ((kPrimaryTableSize - 1) << kCacheIndexShift);
 }
 

src/ic/stub-cache.h

@@ -84,8 +84,7 @@ class StubCache {
   static const int kSecondaryTableBits = 9;
   static const int kSecondaryTableSize = (1 << kSecondaryTableBits);
 
-  // Some magic number used in primary and secondary hash computations.
-  static const int kPrimaryMagic = 0x3d532433;
+  // Some magic number used in the secondary hash computation.
   static const int kSecondaryMagic = 0xb16ca6e5;
 
   static int PrimaryOffsetForTesting(Name* name, Map* map) {