R600/SI: Fix spilling of m0 register

If we have spilled the value of the m0 register, then we need to restore
it with v_readlane_b32 to a regular sgpr, because v_readlane_b32 can't
write to m0.

v_readlane_b32 can't write to m0, so
R600/SI: Mark s_movk_i32 as rematerializable