[InstCombine] Sink pure instructions down to return and unreachable blocks

If the only user of `Instr` is in a return or unreachable block, we can
sink `Instr` to the`User` safely (unless it reads/writes memory).
Return or unreachable blocks are guaranteed to execute zero
or one time, and `Instr` always dominates `User`, so they either will
be executed together (execution of `User` always implies execution
of `Instr`) or not executed at all.

Differential Revision: https://reviews.llvm.org/D80120
Reviewed By: asbirlea, jdoerfert