Optimize away cases like:

Optimize away cases like:
        %cast109 = cast uint %cast212 to short          ; <short> [#uses=1]
        %cast214 = cast short %cast109 to uint          ; <uint> [#uses=1]
        %cast215 = cast uint %cast214 to short          ; <short> [#uses=1]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@3897 91177308-0d34-0410-b5e6-96231b3b80d8