[InstCombine] Fix a code-sinking bug after D73832/f1a9efabcb9b

- UserParent = PN->getIncomingBlock(*I->use_begin());
+ UserParent = PN->getIncomingBlock(*SingleUse);

The first use of I may be droppable (llvm.assume).

When compiling llvm/lib/IR/AutoUpgrade.cpp with a bootstrapped clang
with ThinLTO with minimized bitcode files, I see such a case in
the function _ZN4llvm20UpgradeIntrinsicCallEPNS_8CallInstEPNS_8FunctionE

  clang -c -fthinlto-index=AutoUpgrade.o.thinlto.bc AutoUpgrade.bc -O3

Unfortunately it is really difficult to get a minimized reproduce.

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -3505,7 +3505,7 @@ bool InstCombiner::run() {
 
         // Get the block the use occurs in.
         if (PHINode *PN = dyn_cast<PHINode>(UserInst))
-          UserParent = PN->getIncomingBlock(*I->use_begin());
+          UserParent = PN->getIncomingBlock(*SingleUse);
         else
           UserParent = UserInst->getParent();