[VPlan] Expand SCEV casts in VPSCEVExpander. (#206371)
Add support for expanding scTruncate, scZeroExtend and scSignExtend SCEV
expressions to Trunc/ZExt/SExt VPInstructions in
VPSCEVExpander::tryToExpand.
PR: https://github.com/llvm/llvm-project/pull/206371
