[OpaquePtr][LoopAccessAnalysis] Support opaque pointers

Previously we relied on the pointee type to determine what type we need
to do runtime pointer access checks.

With opaque pointers, we can access a pointer with more than one type,
so now we keep track of all the types we're accessing a pointer's
memory with.

Also some other minor getPointerElementType() removals.

Reviewed By: #opaque-pointers, nikic

Differential Revision: https://reviews.llvm.org/D119047