[PowerPC] Replace the PPCISD:: SExtVElems with ISD::SIGN_EXTEND_INREG to leverage the combine rules

The PPCISD::SExtVElems was added by commit https://reviews.llvm.org/D34009. However,
we have another ISD node ISD::SIGN_EXTEND_INREG that perfectly match the semantics
of SExtVElems. And the DAGCombiner has some combine rules for SIGN_EXTEND_INREG
that produce better code.

Differential Revision: https://reviews.llvm.org/D70771