[InstCombine] Sink instructions with multiple users in a successor block.

This patch tries to sink instructions when they are only used in a successor block.

This is a further enhancement patch based on Anna's commit:
D109700, which allows sinking an instruction having multiple uses in a single user.

In this patch, sink instructions with multiple users in a single successor block will be supported.
It could fix a known issue from rust:
  https://github.com/rust-lang/rust/issues/51346#issuecomment-394443610

Reviewed By: nikic, reames

Differential Revision: https://reviews.llvm.org/D121585