[mips] Fix crash on recursive using of .set

Switch to the `MCParserUtils::parseAssignmentExpression` for parsing
assignment expressions in the `.set` directive reduces code and allows
to print an error message instead of crashing in case of incorrect
recursive using of the `.set`.

Fix for the bug https://bugs.llvm.org/show_bug.cgi?id=41053.

Differential Revision: http://reviews.llvm.org/D59452

llvm-svn: 356461