Revert D123198 "[BuildLibCalls] Introduce getOrInsertLibFunc() for use when building libcalls."

test/Transforms/InstCombine/pr39177.ll failed in a -DLLVM_USE_SANITIZER=Undefined build.
```
lib/Transforms/Utils/BuildLibCalls.cpp:1217:17: runtime error: reference binding to null pointer of type 'llvm::Function'
```
`Function &F = *M->getFunction(Name);`

This reverts commit 0f8c6267.