[SROA] Teach promote to register about droppable instructions

This is the second of two patches to address PR46753. We basically allow
SROA to promote allocas that are used in doppable instructions, for
now that means `llvm.assume`. The (transitive) uses are replaced by
`undef` in the droppable instructions.

See also D83976.

Reviewed By: Tyker

Differential Revision: https://reviews.llvm.org/D83978