[AMDGPU] Add liverange split instructions into BB Prolog
The COPY inserted for liverange split during sgpr-regalloc
pipeline currently breaks the BB prolog during the subsequent
vgpr-regalloc phase while spilling and/or splitting the vector
liveranges. This patch fixes it by correctly including the
the LR split instructions during sgpr-regalloc and wwm-regalloc
pipelines into the BB prolog.