[LAA] Remove RuntimeCheckingPtrGroup::RtCheck member (NFC).

This patch removes RtCheck from RuntimeCheckingPtrGroup to make it
possible to construct RuntimeCheckingPtrGroup objects without a
RuntimePointerChecking object. This should make it easier to
re-use the code to generate runtime checks, e.g. in D102834.

RtCheck was only used to access the pointer info for a given index.
Instead, the start and end expressions can be passed directly.

For code-gen, we also need to know the address space to use. This can
also be explicitly passed at construction.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D105481