Move two members out of MIRGraph::temp_::ssa.

It turns out they are used outside the SSA transformation
by the x86 back-end.

This is a partial revert of
  https://android-review.googlesource.com/120571

Change-Id: Ia5cb2988ab0625d8519901124bd4fc184d5f0886
Merge "Move two members out of MIRGraph::temp_::ssa."