Separate popcnt and sse4.2 feature control somewhat to match gcc behavior.

Enabling sse4.2 will implicitly enable popcnt unless popcnt is explicitly disabled.
Disabling sse4.2 will not disable popcnt if popcnt is explicitly enabled.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190387 91177308-0d34-0410-b5e6-96231b3b80d8