[Mips] Instruction `sc` now accepts symbol as an argument

Function MipsAsmParser::expandMemInst() did not properly handle
instruction `sc` with a symbol as an argument because first argument
would be counted twice. We add additional checks and handle this case
separately.

Patch by Mirko Brkusanin.

Differential Revision: https://reviews.llvm.org/D64252

llvm-svn: 368160