[ADT] Rewrite the StringRef::find implementation to be simpler, clearer,

and tremendously less reliant on the optimizer to fix things.

The code is always necessarily looking for the entire length of the
string when doing the equality tests in this find implementation, but it
previously was needlessly re-checking the size each time among other
annoyances.

By writing this so simply an ddirectly in terms of memcmp, it also is
about 8x faster in a debug build, which in turn makes FileCheck about 2x
faster in 'ninja check-llvm'. This saves about 8% of the time for
FileCheck-heavy parts of the test suite like the x86 backend tests.

llvm-svn: 247269