Revert "[SCEV][IndVarSimplify] insert point should not be block front."

This reverts commit f1efb8bb.

Reverted because it doesn't correctly update the pass return status, see

http://lab.llvm.org:8011/builders/llvm-clang-x86_64-expensive-checks-debian/builds/9441/steps/test-check-all/logs/FAIL%3A%20LLVM%3A%3Awiden-i32-i8ptr.ll