[PowerPC] Set SubRegIndex offset for sub_vsx1/sub_pair1

We defined SubRegIndex for 256/512 regs,
but we did not set the offset for higher part,
so the offset of lower and higher part are the same.
This may cause problem in assessing ranges of SubReg,
it is great that this haven't affected any testcases,
but I think we should fix it to avoid hidden bugs in the future.

Reviewed By: bsaleil, #powerpc

Differential Revision: https://reviews.llvm.org/D92864