[mips] Optimize long branch for MIPS64 by removing %higher and %highest.
%higher and %highest can have non-zero values only for offsets greater
than 2GB, which is highly unlikely, if not impossible when compiling a
single function. This makes long branch for MIPS64 3 instructions smaller.

Differential Revision: http://llvm-reviews.chandlerc.com/D3281.diff