[GlobalISel] IRTranslator: Use RAIIMFObsDelInstaller (#102379)

Similar to #102156.

runOnMachineFunction() installs the Observer correctly manually.

finishPendingPhis() doesn't install into the MF, but this currently
can't cause issues because DILocationVerifier doesn't care about changed
instructions.

Switch to RAIIMFObsDelInstaller in both places for consistency.

Author: tobias-stadler

llvm/lib/CodeGen/GlobalISel/IRTranslator.cpp

@@ -3339,7 +3339,7 @@ void IRTranslator::finishPendingPhis() {
 #ifndef NDEBUG
   DILocationVerifier Verifier;
   GISelObserverWrapper WrapperObserver(&Verifier);
-  RAIIDelegateInstaller DelInstall(*MF, &WrapperObserver);
+  RAIIMFObsDelInstaller ObsInstall(*MF, WrapperObserver);
 #endif // ifndef NDEBUG
   for (auto &Phi : PendingPHIs) {
     const PHINode *PI = Phi.first;
@@ -3966,8 +3966,7 @@ bool IRTranslator::runOnMachineFunction(MachineFunction &CurMF) {
     DILocationVerifier Verifier;
     WrapperObserver.addObserver(&Verifier);
 #endif // ifndef NDEBUG
-    RAIIDelegateInstaller DelInstall(*MF, &WrapperObserver);
-    RAIIMFObserverInstaller ObsInstall(*MF, WrapperObserver);
+    RAIIMFObsDelInstaller ObsInstall(*MF, WrapperObserver);
     for (const BasicBlock *BB : RPOT) {
       MachineBasicBlock &MBB = getMBB(*BB);
       // Set the insertion point of all the following translations to