[PowerPC] On the A2, popcnt[dw] are very slow

The A2 cores support the popcntw/popcntd instructions, but they're microcoded,
and slower than our default software emulation. Specifically, popcnt[dw] take
approximately 74 cycles, whereas our software emulation takes only 24-28
cycles.

I've added a new target feature to indicate a slow popcnt[dw], instead of just
removing the existing target feature from the a2/a2q processor models, because:
  1. This allows us to return more accurate information via the TTI interface
     (I recognize that this currently makes no practical difference)
  2. Is hopefully easier to understand (it allows the core's features to match
     its manual while still having the desired effect).

llvm-svn: 264600