[CostModel] Model all `extractvalue`s as free.

Summary:
As disscussed in https://reviews.llvm.org/D65148#1606412,
`extractvalue` don't actually generate any code,
so we should treat them as free.

Reviewers: craig.topper, RKSimon, jnspaulsson, greened, asb, t.p.northover, jmolloy, dmgreen

Reviewed By: jmolloy

Subscribers: javed.absar, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66098

llvm-svn: 370339