Rename TemplateArgument::getTypeForDecl to getParamTypeForDecl for clarity

Code review feedback from Richard Smith on r219900.
R600/SI: Use early return instead of checking condition twice

Any commutable instruction will have at least src1.
R600/SI: Allow comuting fp immediates
R600/SI: Cleanup code with ChangeToFPImmediate
R600/SI: Fix general commuting breaking src mods

The generic code trying to use findCommutedOpIndices won't
understand that it needs to swap the modifier operands also,
so it should fail if they are set.
R600/SI: Simplify code with hasModifiersSet
R600/SI: Allow commuting with source modifiers