AMDGPU: Delete dead code

There is no context where s_mov_b64 is emitted
and could potentially be moved to the VALU.
It is currently only emitted for materializing
immediates, which can't be dependent on vector sources.

The immediate splitting is already done when selecting
constants. I'm not sure what contexts if any the register
splitting would have been used before.

Also clean up using s_mov_b64 in place of v_mov_b64_pseudo,
although this isn't required and just skips the extra step
of eliminating the copy from the SReg_64.

llvm-svn: 246080