Use llvm::Log2_32 and llvm::Log2_64 instead of llvm::findLastSet (NFC)

For a nonzero argument, llvm::findLastSet(x) is equivalent to
llvm::Log2_32(x) or llvm::Log2_64(x).  None of the calls to
llvm::findLastSet in this patch relies on llvm::findLastSet's ability
to return std::numeric_limits<T>::max() on input 0.