Revert "[IR] Ignore bitcasts of function pointers which are only used as...

Revert "[IR] Ignore bitcasts of function pointers which are only used as callees in callbase instruction"

This reverts commit 167ea67d

This causes a bunch of build failures:
* http://lab.llvm.org:8011/#/builders/121/builds/6287
* http://green.lab.llvm.org/green/job/clang-stage1-RA/19915