Replace llvm.SI.export with llvm.amdgcn.exp and llvm.amdgcn.exp.compr.. The compr bit is separate because it changes the number and type of the operands.
This is a WIP patch because I'm not sure whether the en mask should be an explicit operand, or whether it should be computed from which inputs are undef. Right now dead code is emitted for disabled inputs.
// src3