Replace PPC instruction-size code with MCInstrDesc getSize

As part of the cleanup done to enable the disassembler, the PPC instructions
now have a valid Size description field. This can now be used to replace some
custom logic in a few places to compute instruction sizes.

Patch by David Wiberg!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200623 91177308-0d34-0410-b5e6-96231b3b80d8