[InstCombine] Remove weaker fence adjacent to a stronger fence

We have an instCombine rule to remove identical consecutive fences.
We can extend this to remove weaker fences when we have consecutive stronger
fence.

As stated in the LangRef, a fence with a stronger ordering also implies
ordering weaker than itself: "A fence which has seq_cst ordering, in addition to
having both acquire and release semantics specified above, participates in the
global program order of other seq_cst operations and/or fences."

Reviewed-By: reames

Differential Revision: https://reviews.llvm.org/D118607