[AggressiveInstCombine] Inline strcmp/strncmp (#89371)

Inline calls to strcmp(s1, s2) and strncmp(s1, s2, N), where N and
exactly one of s1 and s2 are constant.

For example:

```c
int res = strcmp(s, "ab");
```

is converted to

```c
int res = (int)s[0] - (int)'a';
if (res != 0)
  goto END;
res = (int)s[1] - (int)'b';
if (res != 0)
  goto END;
res = (int)s[2] - (int)'\0';
END:
```

Ported from a similar gcc feature [Inline strcmp with small constant
strings](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78809).

llvm/include/llvm/Analysis/ValueTracking.h

@@ -117,6 +117,8 @@ bool isKnownToBeAPowerOfTwo(const Value *V, const DataLayout &DL,
                             const DominatorTree *DT = nullptr,
                             bool UseInstrInfo = true);
 
+bool isOnlyUsedInZeroComparison(const Instruction *CxtI);
+
 bool isOnlyUsedInZeroEqualityComparison(const Instruction *CxtI);
 
 /// Return true if the given value is known to be non-zero when defined. For

llvm/include/llvm/Transforms/AggressiveInstCombine/AggressiveInstCombine.h

@@ -8,7 +8,7 @@
 /// \file
 ///
 /// AggressiveInstCombiner - Combine expression patterns to form expressions
-/// with fewer, simple instructions. This pass does not modify the CFG.
+/// with fewer, simple instructions.
 ///
 //===----------------------------------------------------------------------===//
 

llvm/lib/Analysis/ValueTracking.cpp

@@ -252,6 +252,13 @@ bool llvm::haveNoCommonBitsSet(const WithCache<const Value *> &LHSCache,
                                         RHSCache.getKnownBits(SQ));
 }
 
+bool llvm::isOnlyUsedInZeroComparison(const Instruction *I) {
+  return !I->user_empty() && all_of(I->users(), [](const User *U) {
+    ICmpInst::Predicate P;
+    return match(U, m_ICmp(P, m_Value(), m_Zero()));
+  });
+}
+
 bool llvm::isOnlyUsedInZeroEqualityComparison(const Instruction *I) {
   return !I->user_empty() && all_of(I->users(), [](const User *U) {
     ICmpInst::Predicate P;