[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