Use Argument::hasAttribute and AttributeList::ReturnIndex more

This eliminates many extra 'Idx' induction variables in loops over
arguments in CodeGen/ and Target/. It also reduces the number of places
where we assume that ReturnIndex is 0 and that we should add one to
argument numbers to get the corresponding attribute list index.

NFC

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301666 91177308-0d34-0410-b5e6-96231b3b80d8
15 files changed