[SHT_LLVM_BB_ADDR_MAP] Fixes two bugs in decoding of PGOAnalyses in BBAddrMap. (#77139)

We had specified that `readBBAddrMap` will always keep PGOAnalyses and
BBAddrMaps the same length on success.

https://github.com/llvm/llvm-project/blob/365fbbfbcfefb8766f7716109b9c3767b58e6058/llvm/include/llvm/Object/ELFObjectFile.h#L116-L117

It turns out that this is not currently the case when no analyses exist
in a function. No test had caught it.

We also should not append PGOBBEntries when there is no BBFreq or
BrProb.

This patch adds:
* tests that PGOAnalyses and BBAddrMaps are same length even when no
analyses are enabled
* fixes decode so that PGOAnalyses and BBAddrMaps are same length
* updates test to not emit unnecessary PGOBBEntries
* fixes decode to not emit PGOBBEntries when unnecessary