[mips] Free up two values in SubtargetFeatureFlag by folding the redundant IsGP32/IsGP64 into IsGP32bit/IsGP64bit

Summary:
We are currently very close to the 32-bit limit of the current assembler
implementation. This is because there is no way to represent an instruction
that is available in, for example, Mips3 or Mips32. We have to define a
feature bit that represents this.

This patch cleans up a pair of redundant feature bits and slightly postpones the
point we will reach the limit.

Reviewers: zoran.jovanovic, jkolek, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3703