[Attributor] Use knowledge retained in llvm.assume (operand bundles)

This patch integrates operand bundle llvm.assumes [0] with the
Attributor. Most IRAttributes will now look at uses of the associated
value and if there are llvm.assume operand bundle uses with the right
tag we will check if they are in the must-be-executed-context (around
the context instruction). Droppable users, which is currently only
llvm::assume, are handled special in some places now as well.

[0] http://lists.llvm.org/pipermail/llvm-dev/2019-December/137632.html

Reviewed By: uenoku

Differential Revision: https://reviews.llvm.org/D74888