Conversion and handling for vector<T,1>

[llvm-beanz]

Which document does this relate to?
Language Spec

Describe the issue you see with the spec
There are a few problems that have come up during discussion of #311, and the corresponding implementation. DXC has some slightly odd handling of single-element vectors which it would be nice to rectify.

Additional context
Tex suggested trying to make vector<T,1> just an alias of the scalar type T. That will likely break some existing code, but might make things a lot cleaner. The conversion rules would then need to be updated to ensure that we always truncate to scalar instead of vector<T,1>.