[VPlan] Support live-ins without underlying IR in type analysis. (#80723)

A VPlan contains multiple live-ins without underlying IR, like VFxUF or
VectorTripCount. Trying to infer the scalar type of those causes a crash
at the moment.

Update VPTypeAnalysis to take a VPlan in its constructor and assign
types to those live-ins up front. All those live-ins share the type of
the canonical IV.

PR: https://github.com/llvm/llvm-project/pull/80723