[LLVM][Fix] Do not emit debuginfo on vscale or other unknown types

[cbalint13]

This fix instruct LLVM debuginfo to not emit entries for invalid variable types.

Prior to this, it was impossible to build and run a valid TIR program having a flow with scalable vectors.

tvm/src/target/llvm/codegen_llvm.cc", line 2163,
        in tvm::codegen::CodeGenLLVM::VisitStmt_(tvm::tir::LetStmtNode const*)
   AddDebugInformation(value, op->var);
{...} 
File "/home/cbalint/rpmbuild/BUILD/tvm-0.21.0-build/tvm/src/target/llvm/codegen_llvm.cc",  line 2277, 
   in tvm::codegen::CodeGenLLVM::AddDebugInformation(llvm::Value*, tvm::tir::Var const&, llvm::Instruction*)
 auto dbg_dtype = GetDebugType(GetType(tir_var));
{...}

tvm.error.InternalError: Can't fetch the lanes of a scalable vector at a compile time.

This was discovered in PR#18182 by manual checking the TIR kernels.

Notes

• More direct is_void() voidness check, .lanes() property check is bundled with exception throw.
• In case of scalabe vector types do not emit debuginfo, also do not emit debug info for other unknown types.

A testcase now covers this issue for the future.