[Transforms] Do not drop !preserve.access.index metadata

[Transforms] Do not drop !preserve.access.index metadata

Currently, when a GVN or CSE optimization happens,
the llvm.preserve.access.index metadata is dropped.
This caused a problem for BPF AbstructMemberOffset phase
as it relies on the metadata (debuginfo types).

This patch added proper hooks in lib/Transforms to
preserve !preserve.access.index metadata. A test
case is added to ensure metadata is preserved under CSE.

Differential Revision: https://reviews.llvm.org/D65700

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@367769 91177308-0d34-0410-b5e6-96231b3b80d8