Since we use the fact that some uses are droppable in the Attributor we
need to handle them explicitly when we replace uses. As an example, an
assumed dead value can have live droppable users. In those we cannot
replace the value simply by an undef. Instead, we either drop the uses
(via `dropDroppableUses`) or keep them as they are. In this patch we do
both, depending on the situation. For values that are dead but not
necessarily removed we keep droppable uses around because they contain
information we might be able to use later. For values that are removed
we drop droppable uses explicitly to avoid replacement with undef.
The check if globals were accessed was not always working because two
bits are set for NO_GLOBAL_MEM. The new check works also if only on kind
of globals (internal/external) is accessed.
Attributor.cpp became quite big and we need to start provide structure.
The Attributor code is now in Attributor.cpp and the classes derived
from AbstractAttribute are in AttributorAttributes.cpp. Minor changes
were required but no intended functional changes.
We also minimized includes as part of this.
Reviewed By: baziotis
Differential Revision: https://reviews.llvm.org/D76873
Johannes Doerfert