[llvm] [InstCombine] Add user-count bailout to isAllocSiteRemovable (PR #190347)

[Yingwei Zheng via llvm-commits]

================
@@ -3922,8 +3932,25 @@ Instruction *InstCombinerImpl::visitAllocSite(Instruction &MI) {
       F.hasFnAttribute(Attribute::SanitizeAddress))
     KnowInitUndef = false;
 
+  // Skip alloc sites with many direct users -- they are almost never removable
+  // and the transitive user walk in isAllocSiteRemovable is expensive.
+  {
+    unsigned DirectUserCount = 0;
+    for (auto UI = MI.user_begin(), UE = MI.user_end(); UI != UE; ++UI) {
+      if (++DirectUserCount > MaxAllocSiteRemovableUsers)
+        break;
+    }
+    if (DirectUserCount > MaxAllocSiteRemovableUsers)
+      return nullptr;
+  }
----------------
dtcxzyw wrote:

```suggestion
    if (MI.hasNUsesOrMore(MaxAllocSiteRemovableUsers))
      return nullptr;
```

https://github.com/llvm/llvm-project/pull/190347