AMDGPU/GlobalISel: Eliminate SelectVOP3Mods_f32

Trivial type predicates should be moved into the tablegen pattern
itself, and not checked inside complex patterns. This eliminates a
redundant complex pattern, and fixes select source modifiers for
GlobalISel.

I have further patches which fully handle select in tablegen and
remove all of the C++ selection, although it requires the ugliness to
support the entire range of legal register types.