[AMDGPU] Improve codegen of extractelement/insertelement in some cases

This patch improves the codegen of extractelement and insertelement for vector
containing 8 elements. Before, a dag combine transformation was generating a
sequence of 8 select/cmp.
This patch changes the upper limit for this transformation and the movrel
instruction will eventually be used instead. Extractlement/insertelement for
vectors containing less than 8 elements are unchanged.

Differential Revision: https://reviews.llvm.org/D126389