[BPI][NFC] Reuse post dominantor tree from analysis manager when available

Summary: Currenlty BPI unconditionally creates post dominator tree each time. While this is not incorrect we can save compile time by reusing existing post dominator tree (when it's valid) provided by analysis manager.

Reviewers: skatkov, taewookoh, yrouban

Reviewed By: skatkov

Subscribers: hiraditya, steven_wu, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78987