[VPlan] Fix packed replication of struct types (#160274)
I ran into this crash when #158690 caused a loop with a struct call to
be vectorized.
If we have a replicate recipe in a branch-on-mask predicated region
that's used by a widened recipe in another block then it will be packed
together with the other lanes via a VPPredInstPHIRecipe.
If we're replicating a call with a struct return type then we currently
crash. The code that handles structs in packScalarIntoVectorizedValue
seemed to be untested at least on test/Transforms/LoopVectorize.
There's two places that need to be fixed. The poison value that the
scalar is packed into needs to use toVectorizedTy to correctly handle
structs (not to be confused with toVectorTy!)
The other is that VPPredInstPHIRecipe expects its operand to be an
InsertElementInstr when stringing together the different lanes. For
structs this will be an InsertVlaueInstr, and the value for the previous
lane will be at the back of a chain of InsertValueInstrs.
