[RISCV] Cleanup some places that assumed VLMaxSentinel and -1 constant mean the same thing. NFCI

VLMaxSentintel happens to be represented as -1 TargetConstant. A user
provided -1 would be an ISD::Constant. We shouldn't assume that they
are the same thing. I'm still not entirely convinced that we should be
treating -1 from the user as VLMAX.

Also fix one place that failed to use XLenVT for the VLMaxSentinel,
using MVT::i64 in code that only executes on RV32.