[clang] [Clang][RISCV] Attach a range attribute to the vlenb builtin result (PR #219924)

[Luke Lau via cfe-commits]

https://github.com/lukel97 commented:

Should we just emit a vscale intrinsic instead? I think vlenb would be defined then as `vscale * 8 = vlenb`, and it would automatically get all the value tracking information in vplan.

`vscale` isn't really defined for zvl32b but this code uses `llvm::RISCV::RVVBytesPerBlock` which has the same problem anyway. 

https://github.com/llvm/llvm-project/pull/219924