[X86] Initialize FMA3Info directly in its constructor instead of relying on std:...

[X86] Initialize FMA3Info directly in its constructor instead of relying on std::call_once

FMA3Info only exists as a managed static. As far as I know the ManagedStatic construction proccess is thread safe. It doesn't look like we ever access the ManagedStatic object without immediately doing a query on it that would require the map to be populated. So I don't think we're ever deferring the calculation of the tables from the construction of the object.

So I think we should be able to just populate the FMA3Info map directly in the constructor and get rid of all of the initGroupsOnce stuff.

Differential Revision: https://reviews.llvm.org/D48194

llvm-svn: 335064