[VPlan] Move mayHaveSideeffects for FORs check to VPlan.

[fhahn]

Summary

This patch moves the check if sink candidate may have sideeffects to
VPlan. Unfortunately this requires adding logic to try to convert
first-order recurrence phis to induction phis with an assumption. This
is because Legal tries to convert phis to inductions using PSE if they
cannot be converted to a FOR.

The current state of this patch is not ideal, the question is if there's
a better option to handle the fallback to predicated inductions.

[peterwaller-arm]

Are there other parent revisions? The base isn't a valid commit and the patch doesn't apply to main nor commits from the end of January, for example CompareByComesBefore isn't in the patch context but is on main and is old.