[GISel][NFC]: Add unit test for clarifying CSE behavior

Add a unit test that shows how CSE works if we install an observer
at the machine function level and not use the CSEMIRBuilder to build
instructions.

https://reviews.llvm.org/D81625

Author: Aditya Nandakumar

llvm/unittests/CodeGen/GlobalISel/CSETest.cpp

@@ -77,6 +77,25 @@ TEST_F(AArch64GISelMITest, TestCSE) {
   auto Undef0 = CSEB.buildUndef(s32);
   auto Undef1 = CSEB.buildUndef(s32);
   EXPECT_EQ(&*Undef0, &*Undef1);
+
+  // If the observer is installed to the MF, CSE can also
+  // track new instructions built without the CSEBuilder and
+  // the newly built instructions are available for CSEing next
+  // time a build call is made through the CSEMIRBuilder.
+  // Additionally, the CSE implementation lazily hashes instructions
+  // (every build call) to give chance for the instruction to be fully
+  // built (say using .addUse().addDef().. so on).
+  GISelObserverWrapper WrapperObserver(&CSEInfo);
+  RAIIMFObsDelInstaller Installer(*MF, WrapperObserver);
+  MachineIRBuilder RegularBuilder(*MF);
+  RegularBuilder.setInsertPt(*EntryMBB, EntryMBB->begin());
+  auto NonCSEFMul = RegularBuilder.buildInstr(TargetOpcode::G_AND)
+                        .addDef(MRI->createGenericVirtualRegister(s32))
+                        .addUse(Copies[0])
+                        .addUse(Copies[1]);
+  auto CSEFMul =
+      CSEB.buildInstr(TargetOpcode::G_AND, {s32}, {Copies[0], Copies[1]});
+  EXPECT_EQ(&*CSEFMul, &*NonCSEFMul);
 }
 
 TEST_F(AArch64GISelMITest, TestCSEConstantConfig) {