[CodeGenPrepare] Fix isIVIncrement (PR49466)

In the NFC commit 8d835f42, the check for `!L` is
moved to a separate function `getIVIncrement` which, instead of using `BO->getParent()`,
uses `PN->getParent()`. However, these two basic blocks are not necessarily the same.

https://bugs.llvm.org/show_bug.cgi?id=49466 demonstrates a case where `PN` is contained in
a loop while `BO` is not, causing the null-pointer dereference in `L->getLoopLatch()`.

This patch checks whether both `BO` and `PN` belong to the same loop before entering `getIVIncrement`.

Reviewed By: mkazantsev

Differential Revision: https://reviews.llvm.org/D98144