Add argmononly attribute to strlen and wcslen, i.e. they only read memory (string) passed to them.

Summary:
This allows strlen to be moved out of the loop in case its argument is
not modified in the loop in LICM.

Reviewers: hfinkel, davide, sanjoy, dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34323

llvm-svn: 305641