[PowerPC][Future] Branch Distance Estimation For Prefixed Instructions

By adding the prefixed instructions the branch distances are no longer
computed correctly. Since prefixed instructions cannot cross a 64 byte
boundary we have to assume that a prefixed instruction may have a nop
prepended to it. This patch tries to take that nop into consideration
when computing the size of basic blocks.

Differential Revision: https://reviews.llvm.org/D72572