[RISCVAsmParser] Allow a SymbolRef operand to be a complex expression

So that instructions like `lla a5, (0xFF + end) - 4` (supported by GNU as) can
be parsed.

Add a missing test that an operand like `foo + foo` is not allowed.

Reviewed By: jrtc27

Differential Revision: https://reviews.llvm.org/D92293