[Mem2Reg] Teach promote to register about droppable instructions

This is the first of two patches to address PR46753. We basically allow
mem2reg to promote allocas that are used in doppable instructions, for
now that means `llvm.assume`. The uses of the alloca (or a bitcast or
zero offset GEP from there) are replaced by `undef` in the droppable
instructions.

Reviewed By: Tyker

Differential Revision: https://reviews.llvm.org/D83976