Implement the IsDigitOptimization for simplifying calls to the isdigit

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@21688 91177308-0d34-0410-b5e6-96231b3b80d8