BPF: fix a bug in IRPeephole pass

Commit 009f3a89 ("BPF: remove intrindics @llvm.stacksave()
and @llvm.stackrestore()") implemented IRPeephole pass to remove
llvm.stacksave()/stackrestore() instrinsics.
Buildbot reported a failure:
  UNREACHABLE executed at ../lib/IR/LegacyPassManager.cpp:1445!
which is:
  llvm_unreachable("Pass modifies its input and doesn't report it");

The code has changed but the implementation didn't return true
for changing. This patch fixed this problem.