[SimplifyLibCalls] Fix memchr misoptimization (#106121)

The `ch` argument of memcmp should be truncated to `unsigned char`
before using it in comparisons. This didn't happen on all code paths.
The following program miscompiled at -O1 and higher:

```C++
#include <cstring>
#include <iostream>

char ch = '\x81';

int main() {
    bool found = std::strchr("\x80\x81\x82", ch) != nullptr;
    std::cout << std::boolalpha << found << '\n';
}
```

1 files changed, 4 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
index fb2efe5..1e6dc88 100644
--- a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -1454,10 +1454,12 @@ Value *LibCallSimplifier::optimizeMemChr(CallInst *CI, IRBuilderBase &B) {
     if (NonContRanges > 2)
       return nullptr;
 
+    // Slice off the character's high end bits.
+    CharVal = B.CreateTrunc(CharVal, B.getInt8Ty());
+
     SmallVector<Value *> CharCompares;
     for (unsigned char C : SortedStr)
-      CharCompares.push_back(
-          B.CreateICmpEQ(CharVal, ConstantInt::get(CharVal->getType(), C)));
+      CharCompares.push_back(B.CreateICmpEQ(CharVal, B.getInt8(C)));
 
     return B.CreateIntToPtr(B.CreateOr(CharCompares), CI->getType());
   }