[CodeGen] Add lifetime markers to the list of meta-instructions.

Summary:
Since the lifetime markers are metadata instructions, they should probably be treated as such by the isMetaInstruction predicate.
There was no issue that provoked this change, I just ran across it while investigating another issue.

Reviewers: aprantl, MatzeB

Reviewed By: aprantl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D43111

llvm-svn: 324771