[llvm] [CodeGen, CHERI] Add capability types to MVT. (PR #156616)

[David Chisnall via llvm-commits]

davidchisnall wrote:

> This is definitely quite similar in terms of the constraints that are imposed. Historically this MVT was actually called iFATPTR128

For historical context: I chose the name iFATPTR{size} originally because I expected the code at the SelectionDAG layer to be agnostic to the CHERI and for this to be portable across any things that had fat pointers.  The extra CHERI semantics were intended to be contained entirely in the back end.

https://github.com/llvm/llvm-project/pull/156616