PowerPC/SPE: Add phony registers for high halves of SPE SuperRegs

PowerPC/SPE:  Add phony registers for high halves of SPE SuperRegs

The intent of this patch is to make upper halves of SPE SuperRegs(s0,..,s31)
as artificial regs, similar to how X86 has done it.
And emit store /reload instructions for the required halves.

PR : https://github.com/llvm/llvm-project/issues/57307

Reviewed By: jhibbits

Differential Revision: https://reviews.llvm.org/D152437