[AMDGPU] Divergence driven instruction selection. Part 1.

    Summary: This change is the first part of the AMDGPU target description
    change. The aim of it is the effective splitting the vector and scalar
    flows at the selection stage. Selection uses predicate functions based
    on the framework implemented earlier - https://reviews.llvm.org/D35267

    Differential revision: https://reviews.llvm.org/D52019

    Reviewers: rampitec

llvm-svn: 342719