[RISCV] Fix uninitialized variable after call to evaluateConstantImm

For llvm/test/MC/RISCV/rv64i-aliases-invalid.s, UBSan reports:

lib/Target/RISCV/AsmParser/RISCVAsmParser.cpp:371:9: runtime error:
load of value 3879186881, which is not a valid value for type
'RISCVMCExpr::VariantKind'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior
lib/Target/RISCV/AsmParser/RISCVAsmParser.cpp:371:9 in

It turns out that evaluateConstantImm does not set `VK` and it remains
unitialized when doing comparisons in `isImmXLenLI()`.

Differential Revision: https://reviews.llvm.org/D65347

llvm-svn: 367230