Correct the PPC A2 misprediction penalty

The manual states that there is a minimum of 13 cycles from when the
mispredicted branch is issued to when the correct branch target is
issued.
Enable early if conversion on PPC

On cores for which we know the misprediction penalty, and we have
the isel instruction, we can profitably perform early if conversion.
This enables us to replace some small branch sequences with selects
and avoid the potential stalls from mispredicting the branches.

Enabling this feature required implementing canInsertSelect and
insertSelect in PPCInstrInfo; isel code in PPCISelLowering was
refactored to use these functions as well.