[SimpleLoopUnswitch] Drop uses of instructions before block deletion

Currently if instructions defined in a block are used in unreachable
blocks and SimpleLoopUnswitch attempts deleting the block, it triggers
assertion "Uses remain when a value is destroyed!".
This patch fixes it by replacing all uses of instructions from BB with
undefs before BB deletion.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D80551