[SVE][CodeGen] Scalable vector MVT size queries
- Implements scalable size queries for MVTs, split out from D53137.
- Contains a fix for FindMemType to avoid using scalable vector type to contain non-scalable types.
- Explicit casts for several places where implicit integer sign changes or promotion from 32 to 64 bits caused problems.
- CodeGenDAGPatterns will treat scalable and non-scalable vector types as different.
Reviewers: greened, cameron.mcinally, sdesmalen, rovka
Reviewed By: rovka
Differential Revision: https://reviews.llvm.org/D66871