Implement the IsDigitOptimization for simplifying calls to the isdigit

library function:
  isdigit(chr) -> 0 or 1 if chr is constant
  isdigit(chr) -> chr - '0' <= 9 otherwise

Although there are many calls to isdigit in llvm-test, most of them are
compiled away by macros leaving only this:

2 MultiSource/Applications/hexxagon

llvm-svn: 21688