[mips] Split mem_msa into range checked mem_simm10 and mem_simm10_lsl[123]

Summary:
Also, made test_mi10.s formatting consistent with the majority of the
MC tests.

Reviewers: vkalintiris

Subscribers: dsanders, llvm-commits

Differential Revision: http://reviews.llvm.org/D18435

llvm-svn: 265014