[llvm] [VPlan] Use VPInstructionWithType for uniform casts. (PR #140623)

[via llvm-commits]

================
@@ -866,7 +867,8 @@ bool VPInstruction::isVectorToScalar() const {
 }
 
 bool VPInstruction::isSingleScalar() const {
-  return getOpcode() == Instruction::PHI || isScalarCast();
----------------
ayalz wrote:

Should `VPRecipeBase::isScalarCast()` be removed, replacing its remaining users with code checking for VPInstructions with cast opcode instead?

https://github.com/llvm/llvm-project/pull/140623