cmd/compile: multiple memories live at block start [1.20 backport]

@randall77 requested issue #59973 to be considered for backport to the next 1.20 minor release.

> @gopherbot Please open backport issues.
> 
> This bug can lead to compiler crashes which would be hard to work around.
> I'm not sure if it could lead to miscompilation. But maybe?
> 
> It is `GOAMD64=v3` only.
