[GISel]: Relax opcode checking at the top level to enable CSE

[aditya_nandakumar]

Summary

Loosen the restriction on what kinds of opcodes can be CSEd as targets may want to CSE some generic target specific pseudos.
NFC as far as this change is concerned as CSEConfig still pretty much is a subset of this check.

llvm/lib/CodeGen/GlobalISel/CSEInfo.cpp

 void GISelCSEInfo::handleRecordedInsts() {
   while (!TemporaryInsts.empty()) {
     auto *MI = TemporaryInsts.pop_back_val();
     handleRecordedInst(MI);
   }
 }
 
 bool GISelCSEInfo::shouldCSE(unsigned Opc) const {
-  // Only GISel opcodes are CSEable
-  if (!isPreISelGenericOpcode(Opc))
-    return false;
   assert(CSEOpt.get() && "CSEConfig not set");
   return CSEOpt->shouldCSEOpc(Opc);
 }
 
 void GISelCSEInfo::erasingInstr(MachineInstr &MI) { handleRemoveInst(&MI); }
 void GISelCSEInfo::createdInstr(MachineInstr &MI) { recordNewInstruction(&MI); }
 void GISelCSEInfo::changingInstr(MachineInstr &MI) {
   // For now, perform erase, followed by insert.