Revert r254897 "[mips][microMIPS] Implement LH, LHE, LHU and LHUE instructions"

Commited patch was intended to implement LH, LHE, LHU and LHUE instructions.
After commit test-suite failed with error message in the form of:
fatal error: error in backend: Cannot select: t124: i32,ch = load<LD2[%d](tbaa=<0x94acc48>), sext from i16> t0, t2, undef:i32
For that reason I decided to revert commit r254897 and make new patch which besides implementation and standard regression tests will also have dedicated tests (CodeGen) for the above error. 

llvm-svn: 255109