AMDGPU/GlobalISel: Default to using TableGen'd instruction selector

[tstellar]

Summary

We can select all instructions that are marked as legal in a full piglit run,
so now is a good time to make the TableGen'd instruction selector default
for all opcodes. This is NFC for a full piglit run, which is why there are
no tests.

[nhaehnle]

LGTM

lib/Target/AMDGPU/AMDGPUInstructionSelector.cpp

bool AMDGPUInstructionSelector::select(MachineInstr &I,
   if (!isPreISelGenericOpcode(I.getOpcode())) {
     if (I.isCopy())
       return selectCOPY(I);
     return true;
   }
 
   switch (I.getOpcode()) {
   default:
-    break;
-  case TargetOpcode::G_ASHR:
-  case TargetOpcode::G_SITOFP:
-  case TargetOpcode::G_FMUL:
-  case TargetOpcode::G_FADD:
-  case TargetOpcode::G_FPTOUI:
-  case TargetOpcode::G_OR:
     return selectImpl(I, CoverageInfo);
   case TargetOpcode::G_ADD:
     return selectG_ADD(I);
   case TargetOpcode::G_BITCAST:
     return selectCOPY(I);
   case TargetOpcode::G_CONSTANT:
   case TargetOpcode::G_FCONSTANT:
     return selectG_CONSTANT(I);