[LLDB] Change RegisterValue::GetAsMemoryData to const RegisterInfo&

Most of the paths to this never passed nullptr intentionally. Those
that possibly could have were assuming it was not null elsehwere,
so would have crashed.

I've added asserts in those cases.

At least one case was relying on GetAsMemoryData to return an error
when it was given nullptr. So I've hoisted that error setting code
out into the caller.

Depends on D134963

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D134965