[AMDGPU] Improve code size cost model

Summary:
Added estimation for zero size insertelement, extractelement
and llvm.fabs operators.
Updated inline/unroll parameters default values.

Reviewers: rampitec, arsenm

Reviewed By: arsenm

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68881

llvm-svn: 375109