[PowerPC] Refactor popcnt[dw] target features

[PowerPC] Refactor popcnt[dw] target features

Instead of using two feature bits, one to indicate the availability of the
popcnt[dw] instructions, and another to indicate whether or not they're fast,
use a single enum. This allows more consistent control via target attribute
strings, and via Clang's command line.

llvm-svn: 264690