Re-commit r357452 (take 2): "SimplifyCFG SinkCommonCodeFromPredecessors: Also...

Re-commit r357452 (take 2): "SimplifyCFG SinkCommonCodeFromPredecessors: Also sink function calls without used results (PR41259)"

This was reverted in r360086 as it was supected of causing mysterious test
failures internally. However, it was never concluded that this patch was the
root cause.

> The code was previously checking that candidates for sinking had exactly
> one use or were a store instruction (which can't have uses). This meant
> we could sink call instructions only if they had a use.
>
> That limitation seemed a bit arbitrary, so this patch changes it to
> "instruction has zero or one use" which seems more natural and removes
> the need to special-case stores.
>
> Differential revision: https://reviews.llvm.org/D59936

llvm-svn: 361811