[RISCV] Fixed Assertion`Kind == Immediate && "Invalid type access!"' failed.

Summary:
Missing check for isImm() in some Immediate classes.

This bug was uncovered by a LLVM MC Assembler Protocol Buffer Fuzzer
for the RISC-V assembly language.

Reviewers: hiraditya, asb

Reviewed By: hiraditya, asb

Subscribers: llvm-commits, hiraditya, kito-cheng, shiva0217, rkruppe, asb, rbar, johnrusso, simoncook, sabuasal, niosHD, zzheng, edward-jones, mgrang, rogfer01, MartinMosbeck, brucehoult, the_o, PkmX, jocewei

Differential Revision: https://reviews.llvm.org/D50797

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@340674 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed