[mips] Range check simm7.

Summary:
Also renamed li_simm7 to li16_imm since it's not a simm7 and has an unusual
encoding (it's a uimm7 except that 0x7f represents -1).

Reviewers: vkalintiris

Subscribers: dsanders, llvm-commits

Differential Revision: http://reviews.llvm.org/D18145

llvm-svn: 264056