[InstCombine] Remove an unnecessary dyn_cast to Instruction and a switch over two opcodes. Just dyn_cast to the specific instruction classes individually. NFC

Change the helper methods to take the more specific class as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@311527 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed