[InstCombine] Fix a code-sinking bug after D73832/f1a9efab

- 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.