[CodeGen] Check FoldConstantArithmetic result before using it.

Fixes PR24602: r245689 introduced an unguarded use of
SelectionDAG::FoldConstantArithmetic, which returns 0 when it fails
because of opaque (hoisted) constants.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246217 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed