[mips] PR34083 - Wimplicit-fallthrough warning in MipsAsmParser.cpp

[mips] PR34083 - Wimplicit-fallthrough warning in MipsAsmParser.cpp

Assert that a binary expression is actually a binary expression,
rather than potientially incorrectly attempting to handle it as a
unary expression.

This resolves PR34083.

Thanks to Simonn Pilgrim for reporting the issue!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310460 91177308-0d34-0410-b5e6-96231b3b80d8