[JumpThreading] Change a dyn_cast that is already protected by an isa check to a static cast. Combine the with another static cast. NFC

Differential Revision: https://reviews.llvm.org/D32874

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302197 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed