[tilegx] Fix pipeline issue in SubStringStub::Generate.
[tilegx] Fix two other reg collision bugs.

  frame scope epilogue will use "at" implicitly.
[tilegx] Unify our temp reg name to "at" and "at2", remove all "tt" and "tt2".

  This could easy our debug work.
[tilegx] Fix kSafepointRegisterStackIndexMap.
[tilegx] Fixed several a0/v0 collision.