Fix Intrinsic::getType not working with vararg

VarArg Intrinsic functions are encoded with "void" type as the last
argument. Now Intrinsic::getType can correctly return all the intrinsic
function type.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220205 91177308-0d34-0410-b5e6-96231b3b80d8