Code cleanup on membership block loop

[eileenmcnaughton]

Overview

Code tidy up from #13815

Before

Code more confusing

After

Code less confusing

Technical Details

This is a reviewer's commit from #13815 - that PR had 2 components

1. code cleanup
2. a functional change (fixed one flow, broke another)

Having had to delve fairly deeply to make sense of this I was convinced the code cleanup was a positive improvement & am pulling it out & merging it. I have concerns about the functional change :-(

Comments

[civibot]

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[eileenmcnaughton]

merge on pass as a reviewer's commit