Merge clang's isRepeatedBytePattern with LLVM's isBytewiseValue

Summary:
his code was in CGDecl.cpp and really belongs in LLVM's isBytewiseValue. Teach isBytewiseValue the tricks clang's isRepeatedBytePattern had, including merging undef properly, and recursing on more types.

clang part of this patch: D51752

Subscribers: dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D51751

llvm-svn: 342709