[X86] Move the X86InstrFMA3Info class into the cpp file. Expose only a getFMA3Group...

[X86] Move the X86InstrFMA3Info class into the cpp file. Expose only a getFMA3Group free function. NFCI

The class only exists to hold a DenseMap and is only created as a ManagedStatic. It used to expose a single static method that outside code was expected to use.

This patch moves that static function out of the class and moves it implementation into the cpp file. It can now access the ManagedStatic directly by name without the need for the other static method that accessed the ManagedStatic.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@336055 91177308-0d34-0410-b5e6-96231b3b80d8