isBytewiseValue checks ConstantVector element by element

Summary: Vector of the same value with few undefs will sill be considered "Bytewise"

Reviewers: eugenis, pcc, jfb

Reviewed By: jfb

Subscribers: dexonsmith, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64031

llvm-svn: 365971