AMDGPUPrintfRuntimeBinding.cpp - drop unnecessary casts/dyn_casts. NFCI.

GetElementPtrInst::Create returns a GetElementPtrInst* so we don't need to cast. Similarly IntegerType inherits from the Type base class.

Also, I've used auto* in a few places to cleanup the code.

Helps fix some clang-tidy warnings which saw the dyn_casts and warned that these can return null.