TLI: Microoptimize calls to strlen+memcmp to strncmp.

The strlen+memcmp was hidden in a call to StringRef::operator==. We check if
there are any null bytes in the string upfront so we can simplify the comparison
Small speedup when compiling code with many function calls.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176766 91177308-0d34-0410-b5e6-96231b3b80d8