[AMDGPU] Convert insert_vector_elt into set of selects

This allows to avoid scratch use or indirect VGPR addressing for
small vectors.

Differential Revision: https://reviews.llvm.org/D54606

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@347231 91177308-0d34-0410-b5e6-96231b3b80d8
11 files changed