Fix SILCombine of unchecked_bitwise_cast

This change ensures the insertion point is after any copies/borrows that OSSA rauw may create.

Before this fix we could end up creating invalid SIL where operands do not dominate its instruction.

Similar to swiftlang#64533. Found by inspection.

Author: meg-gupta

lib/SILOptimizer/SILCombiner/SILCombinerCastVisitors.cpp

@@ -900,8 +900,9 @@ visitUncheckedBitwiseCastInst(UncheckedBitwiseCastInst *UBCI) {
 
     OwnershipRAUWHelper helper(ownershipFixupContext, UBCI, Oper);
     if (helper) {
+      SILBuilderWithScope localBuilder(std::next(UBCI->getIterator()), Builder);
       auto replacement = helper.prepareReplacement();
-      auto *transformedOper = Builder.createUncheckedBitwiseCast(
+      auto *transformedOper = localBuilder.createUncheckedBitwiseCast(
           UBCI->getLoc(), replacement, UBCI->getType());
       helper.perform(transformedOper);
       return nullptr;

test/SILOptimizer/sil_combine_nocanonicalize_ossa.sil

@@ -81,3 +81,25 @@ bb0:
   destroy_value %obj : $KlassWithTailAllocatedElems
   return %val : $Builtin.Word
 }
+
+sil @get_native_object : $@convention(thin) () -> @owned Builtin.NativeObject
+
+// CHECK-LABEL: sil [ossa] @bitwise_combines_guaranteed :
+// CHECK: unchecked_ref_cast
+// CHECK-LABEL: } // end sil function 'bitwise_combines_guaranteed'
+sil [ossa] @bitwise_combines_guaranteed : $@convention(thin) () -> @owned Optional<Builtin.NativeObject> {
+bb0:
+  %f = function_ref @get_native_object : $@convention(thin) () -> @owned Builtin.NativeObject
+  %0 = apply %f() : $@convention(thin) () -> @owned Builtin.NativeObject
+  %b = begin_borrow [lexical] %0 : $Builtin.NativeObject
+  br bb1
+
+bb1:
+  %2 = unchecked_trivial_bit_cast %b : $Builtin.NativeObject to $Builtin.Word
+  %3 = unchecked_bitwise_cast %2 : $Builtin.Word to $Optional<Builtin.NativeObject>
+  %c = copy_value %3 : $Optional<Builtin.NativeObject>
+  end_borrow %b : $Builtin.NativeObject
+  destroy_value %0 : $Builtin.NativeObject
+  return %c : $Optional<Builtin.NativeObject>
+}
+