Revert "[FuncSpec] Add Phi nodes to the InstCostVisitor."

This reverts commit 96ff464d.

The test in this change was failing on many buildbots:

https://lab.llvm.org/buildbot/#/builders/164/builds/41292
https://lab.llvm.org/buildbot/#/builders/258/builds/4491
https://lab.llvm.org/buildbot/#/builders/192/builds/3566
https://lab.llvm.org/buildbot/#/builders/123/builds/20411
https://lab.llvm.org/buildbot/#/builders/58/builds/42553
https://lab.llvm.org/buildbot/#/builders/247/builds/7037
https://lab.llvm.org/buildbot/#/builders/139/builds/46259
https://lab.llvm.org/buildbot/#/builders/216/builds/24650
https://lab.llvm.org/buildbot/#/builders/234/builds/12571
https://lab.llvm.org/buildbot/#/builders/232/builds/12574
https://lab.llvm.org/buildbot/#/builders/235/builds/975