[ValueTypes] Add support for scalable EVTs

Summary:
* Remove a bunch of asserts checking for unsupported scalable types and
  add some more now that they are supported.
* Propagate the scalable flag where necessary.
* Add another `EVT::getExtendedVectorVT` method that takes an
  ElementCount parameter.
* Add `EVT::isExtendedScalableVector` and
  `EVT::getExtendedVectorElementCount` - latter is currently unused.

Reviewers: sdesmalen, efriedma, rengolin, craig.topper, huntergr

Reviewed By: efriedma

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75672