NFC: deduplicate isRepeatedBytePattern from clang to LLVM's isBytewiseValue

Summary:
This code was in CGDecl.cpp and really belongs in LLVM. It happened to have isBytewiseValue which served a very similar purpose but wasn't as powerful as clang's version. Remove the clang version, and augment isBytewiseValue to be as powerful so that clang does the same thing it used to.

LLVM part of this patch: D51751

Subscribers: dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D51752

llvm-svn: 342734