[VPlan] Support PHIs as LastInst when inserting scalars in ::get().

At the moment, we create insertelement instructions directly after
LastInst when inserting scalar values in a vector in
VPTransformState::get.

This results in invalid IR when LastInst is a phi, followed by another
phi. In that case, the new instructions should be inserted just after
the last PHI node in the block.

At the moment, I don't think the problematic case can be triggered, but
it can happen once predicate regions are merged and multiple
VPredInstPHI recipes are in the same block (D100260).

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D104188

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -9730,12 +9730,14 @@ Value *VPTransformState::get(VPValue *Def, unsigned Part) {
   }
 
   auto *LastInst = cast<Instruction>(get(Def, {Part, LastLane}));
-
-  // Set the insert point after the last scalarized instruction. This
-  // ensures the insertelement sequence will directly follow the scalar
-  // definitions.
+  // Set the insert point after the last scalarized instruction or after the
+  // last PHI, if LastInst is a PHI. This ensures the insertelement sequence
+  // will directly follow the scalar definitions.
   auto OldIP = Builder.saveIP();
-  auto NewIP = std::next(BasicBlock::iterator(LastInst));
+  auto NewIP =
+      isa<PHINode>(LastInst)
+          ? BasicBlock::iterator(LastInst->getParent()->getFirstNonPHI())
+          : std::next(BasicBlock::iterator(LastInst));
   Builder.SetInsertPoint(&*NewIP);
 
   // However, if we are vectorizing, we need to construct the vector values.