[RISCV][MC] Tighten up checking of sybol operands to lui and auipc

Reject bare symbols and accept only %pcrel_hi(sym) for auipc and %hi(sym) for 
lui. Also test valid operand modifiers in rv32i-valid.s.

Note this is slightly stricter than gas, which will accept either %pcrel_hi or 
%hi for both lui and auipc.

Differential Revision: https://reviews.llvm.org/D51731

llvm-svn: 342486