This reverts commit r360106.
The revisioin causes llvm-tblgen to hang while generating info for
RISCV.td. The root cause might be in the RISCV.td definition but I don't
know enough about this to investigate further.
Command that starts hangning after r360106:
llvm-build/bin/llvm-tblgen -I llvm/include -I llvm/tools/clang/include -I llvm/lib/Target/RISCV -gen-instr-info llvm/lib/Target/RISCV/RISCV.td