FixIrreducible: don't crash when moving a child loop

Summary:
When an irreducible SCC is converted into a new natural loop, existing
loops included in that SCC now become children of the new loop. The
logic that moves these loops from the parent loop to the new loop
invoked undefined behaviour when it modified the container that it was
iterating over. Fixed this by first extracting all the loops that are
to be removed from the parent.

Fixes bug 45623.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D78544