[tilegx] Enable FullCodeGenerator::EffectContext::Plug.
[tilegx] Fix register collision in base line compiler.

  in the implementation of "VisitCountOperation", we need
to update return-reg using arg-reg's value. While arg-reg's
value should not be clobbered. MIPS's return-reg and arg-reg
don't overlap, so it's OK, while for TileGX, both start from
r0, so we need to save/restore arg-reg explictly.

  also fixed one relocation index calculation issue in ic-tilegx.cc.
[tilegx] Cleanup all other return-reg/arg-reg collision issues.

  this and previous commit reduced over one hundred failures.