Add strchr(p, 0) -> p + strlen(p) to SimplifyLibCalls

Add the missing transformation strchr(p, 0) -> p + strlen(p) to SimplifyLibCalls
and remove the ToDo comment.

Reviewer: Duncan P.N. Exan Smith


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200736 91177308-0d34-0410-b5e6-96231b3b80d8