[StackSlotColoring] Fix issue where colors for a StackID are dropped (llvm#138140)

In InitializeSlots, if an interval with a non-zero StackID (A) is
encountered we set All/UsedColors to a size of A + 1. If after this we
process another interval with a non-zero StackID (B), where B < A, then
we resize All/UsedColors to size < A + 1, and lose the BitVector
associated with A.

AFAIK this is a latent bug upstream, but when adding a new TargetStackID
locally I hit a `NextColors[StackID] != -1 && "No more spill slots?"`
assertion due to this issue.

Author: MacDue

llvm/lib/CodeGen/StackSlotColoring.cpp

@@ -287,8 +287,10 @@ void StackSlotColoring::InitializeSlots() {
 
     auto StackID = MFI->getStackID(FI);
     if (StackID != 0) {
-      AllColors.resize(StackID + 1);
-      UsedColors.resize(StackID + 1);
+      if (StackID >= AllColors.size()) {
+        AllColors.resize(StackID + 1);
+        UsedColors.resize(StackID + 1);
+      }
       AllColors[StackID].resize(LastFI);
       UsedColors[StackID].resize(LastFI);
     }