[PATCH] Extend MipsMCExpr class to handle %higher(sym1 - sym2 + const) and %highest(sym1 - sym2 + const) relocations.

[Sasa Stankovic]

  The reason why I extended the MipsMCExpr class to handle not only sym1 - sym2 but also sym1 - sym2 + const expressions was to be able to write test for %higher and %highest.

http://llvm-reviews.chandlerc.com/D3230