Fix returned value for GVN: could return "false" even after modifying the IR

This bug would manifest in some very specific cases where all the following
conditions are fullfilled:

- GVN didn't remove block
- The regular GVN iteration didn't change the IR
- PRE is enabled
- PRE will not split critical edge
- The last instruction processed by PRE didn't change the IR

Because the CallGraph PassManager relies on this returned value to decide
if it needs to recompute a node after the execution of Function passes,
not returning the right value can lead to unexpected results.

Fix for: https://llvm.org/bugs/show_bug.cgi?id=24715

Patch by Wenxiang Qiu <vincentqiuuu@gmail.com>

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 253518