[X86] Replace unnecessary int2double overrides with a better WriteCvtI2PD def

Broadwell, Haswell and SkylakeClient were completely overriding the WriteCvtI2PD defs - we can remove those overrides entirely by replacing the unused WriteCvtI2PD values

There's plenty more of these in the scheduler models - I'm looking at improving warnings in llvm-tblgen to catch them all