[CodeGen] Simplify logic that looks for returned call operands. NFC-ish.

[CodeGen] Simplify logic that looks for returned call operands. NFC-ish.

Use getReturnedArgOperand() instead of rolling our own.  Note that it's
equivalent because there can only be one 'returned' operand.

The existing code was also incorrect: there already was awkward logic to
ignore callee/EH blocks, but operands can now also be operand bundles,
in which case we'll look for non-existent parameter attributes.

Unfortunately, this isn't observable in-tree, as it only crashes when
exercising the regular call lowering logic with operand bundles.
Still, this is a nice small cleanup anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@290905 91177308-0d34-0410-b5e6-96231b3b80d8