[RISCV] Move the creation of VLMaxSentinel to isel. Use X0 during lowering.

The VLMaxSentinel is represented as TargetConstant, but that's included
in isa<ConstantSDNode>. To keep constant VLs and VLMax separate as long
as possible, use the X0 register during lowering and only convert to
VLMaxSentinel during isel.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D118845