[SelectionDAG] Move extension type for ConstantSDNode from getCopyToRegs to...

[SelectionDAG] Move extension type for ConstantSDNode from getCopyToRegs to HandlePHINodesInSuccessorBlocks.

D122053 set the ExtendType for ConstantSDNodes in getCopyToRegs to
ZERO_EXTEND to match assumptions in ComputePHILiveOutRegInfo. PHIs
are probably not the only way ConstantSDNodeNodes can get to
getCopyToRegs.

This patch adds an ExtendType parameter to CopyValueToVirtualRegister and
has HandlePHINodesInSuccessorBlocks pass ISD::ZERO_EXTEND for ConstantInts.
This way we only affect ConstantSDNodes for PHIs.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D122171