[AMDGPU] Order pos exports before param exports

Summary:
Modify export clustering DAG mutation to move position exports
before other exports types.

Reviewers: foad, arsenm, rampitec, nhaehnle

Reviewed By: foad

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79670