[PowerPC][NFC] Reclaim TSFlags bit 6

We removed UseVSXReg flag in https://reviews.llvm.org/D58685
But we did not reclain the bit 6 it was assigned,
this will become confusing and a hole later..
We should reclaim it as early as possible before new bits.

Reviewed By: sfertile

Differential Revision: https://reviews.llvm.org/D72649