[AArch64] Add new target feature to handle cheap as move for Exynos

This feature enables special handling of cheap as move in the existing
custom handling specifically for Exynos processors.

Differential revision: https://reviews.llvm.org/D42387

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@323774 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed