[llvm] [DenseMap] Update combineHashValue (PR #95970)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay updated https://github.com/llvm/llvm-project/pull/95970

>From 2ede7c10f0fce95cfaf5980133a3226b6a56814d Mon Sep 17 00:00:00 2001
From: Fangrui Song <i at maskray.me>
Date: Tue, 18 Jun 2024 11:49:20 -0700
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Created using spr 1.3.5-bogner
---
 llvm/include/llvm/ADT/DenseMapInfo.h | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/llvm/include/llvm/ADT/DenseMapInfo.h b/llvm/include/llvm/ADT/DenseMapInfo.h
index 5b7dce7b53c62..ba05309e4e413 100644
--- a/llvm/include/llvm/ADT/DenseMapInfo.h
+++ b/llvm/include/llvm/ADT/DenseMapInfo.h
@@ -26,17 +26,15 @@ namespace llvm {
 namespace detail {
 
 /// Simplistic combination of 32-bit hash values into 32-bit hash values.
+/// This uses a Murmur3-type mixer "Moremur" from Pelle Evensen.
 static inline unsigned combineHashValue(unsigned a, unsigned b) {
-  uint64_t key = (uint64_t)a << 32 | (uint64_t)b;
-  key += ~(key << 32);
-  key ^= (key >> 22);
-  key += ~(key << 13);
-  key ^= (key >> 8);
-  key += (key << 3);
-  key ^= (key >> 15);
-  key += ~(key << 27);
-  key ^= (key >> 31);
-  return (unsigned)key;
+  uint64_t x = (uint64_t)a << 32 | (uint64_t)b;
+  x ^= x >> 27;
+  x *= 0x3C79AC492BA7B653UL;
+  x ^= x >> 33;
+  x *= 0x1C69B3F74AC4AE35UL;
+  x ^= x >> 27;
+  return (unsigned)x;
 }
 
 } // end namespace detail