[InstCombine] Improve simplify demanded bits worklist management

This fixes a small mistake from D72944: The worklist add should
happen before assigning the new operand, not after.

In case an actual replacement happens, the old operand needs to
be added for DCE. If no actual replacement happens, then old/new
are the same, so it doesn't matter.

This drops one iteration from the annotated test case.

llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp

@@ -87,9 +87,9 @@ bool InstCombiner::SimplifyDemandedBits(Instruction *I, unsigned OpNo,
   Value *NewVal = SimplifyDemandedUseBits(U.get(), DemandedMask, Known,
                                           Depth, I);
   if (!NewVal) return false;
-  U = NewVal;
-  // Add the simplified instruction back to the worklist.
+  // Add the old operand back to the worklist.
   Worklist.addValue(U.get());
+  U = NewVal;
   return true;
 }
 

llvm/test/Transforms/InstCombine/2010-11-01-lshr-mask.ll

@@ -1,4 +1,4 @@
-; RUN: opt -instcombine -S < %s | FileCheck %s
+; RUN: opt -instcombine -instcombine-infinite-loop-threshold=3 -S < %s | FileCheck %s
 
 ; <rdar://problem/8606771>
 define i32 @main(i32 %argc) {