[SCEV][NFC] Replace redundant dyn_cast with cast in getAddExpr

Replace dyn_cast which is ensured by isa just one line above with cast.

Differential Revision: https://reviews.llvm.org/D33231


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303234 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed