Scalar: Avoid dereferencing end() in IndVarSimplify

IndVarSimplify::sinkUnusedInvariants calls
BasicBlock::getFirstInsertionPt on the ExitBlock and moves instructions
before it.  This can return end(), so it's not safe to dereference.  Add
an iterator-based overload to Instruction::moveBefore to avoid the UB.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278886 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed