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

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.

Differential Revision: https://reviews.llvm.org/D78684

lib/CodeGen/GlobalISel/CSEInfo.cpp

@@ -217,9 +217,6 @@ void GISelCSEInfo::handleRecordedInsts() {
 }
 
 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);
 }