[regalloc] Ensure Query::collectInterferringVregs is called before interval iteration

The main part of the patch is the change in RegAllocGreedy.cpp: Q.collectInterferringVregs()
needs to be called before iterating the interfering live ranges.

The rest of the patch offers support that is the case: instead of  clearing the query's
InterferingVRegs field, we invalidate it. The clearing happens when the live reg matrix
is invalidated (existing triggering mechanism).

Without the change in RegAllocGreedy.cpp, the compiler ices.

This patch should make it more easily discoverable by developers that
collectInterferringVregs needs to be called before iterating.

I will follow up with a subsequent patch to improve the usability and maintainability of Query.

Differential Revision: https://reviews.llvm.org/D98232