* Fix the right shift operator which didn't work if the number of bits is exactly 64
* Detect overflow when combining indices
Such large indices usually don't appear in real code, except in internal String operations where (potentially large) integer values are treated as pointers.
Fixes a compiler crash
https://github.com/swiftlang/swift/issues/84372
rdar://160863199
Add a `vectorBase` field kind which corresponds to `vector_base_addr`.
Also, improve `mayOverlap`: it didn't catch the case that the first element of a vector cannot overlap with another element.
