Add a bad char heuristic to StringRef::find.

Add a bad char heuristic to StringRef::find.

Based on Horspool's simplified version of Boyer-Moore. We use a constant-sized table of
uint8_ts to keep cache thrashing low, needles bigger than 255 bytes are uncommon anyways.

The worst case is still O(n*m) but we do a lot better on the average case now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142061 91177308-0d34-0410-b5e6-96231b3b80d8