Set cost of invariant group intrinsics to 0

Summary:
Like with other similar intrinsics, presense of strip or
launder.invariant.group should not change the result of inlining cost.
This is because they are just markers and do not perform any computation.

Reviewers: amharc, rsmith, reames, kuhar

Subscribers: eraman, llvm-commits

Differential Revision: https://reviews.llvm.org/D51814

llvm-svn: 341725