[NVPTX] Eliminate StoreRetval instructions with undef operand

Previously a lot of StoreRetval instructions with undef operand were
generated on NVPTX target when a big struct was returned by value.
It resulted in a lot of unneeded st.param.* instructions in final
assembly. The patch solves the issue by implementing the logic in
NVPTX-specific part of DAG combiner.

Differential Revision: https://reviews.llvm.org/D118973