[mips] Implement %hi(sym1 - sym2) and %lo(sym1 - sym2) expressions

Patch implements %hi(sym1 - sym2) and %lo(sym1 - sym2) expressions for MIPS
by creating target expression class MipsMCExpr.

Patch by Sasa Stankovic.

Differential Revision: http://llvm-reviews.chandlerc.com/D2592