Removes cast instruction checks (in PNaClTranslator.cpp) that used
LLVM utilities to use locally defined methods instead. Remove the need
to call naclbitc::DecodeCastOpcode and CastInst::castIsValid.

Also removes two more calls to convertToLLVMType.

BUG= None
R=jvoung@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/794823002