InstructionSimplify: Refactor foldIdentityShuffles. NFC.

Summary:
Minor refactoring of foldIdentityShuffles() which allows the removal of a
ConstantDataVector::get() in SimplifyShuffleVectorInstruction.

Reviewers: spatel

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32955

Conflicts:
	lib/Analysis/InstructionSimplify.cpp

llvm-svn: 302433