Similar to 806761a7629df268c8aed49657aeccffa6bca449
-mtriple= specifies the full target triple while -march= merely sets the
architecture part of the default target triple (e.g. Windows, macOS),
leaving a target triple which may not make sense.
Therefore, -march= is error-prone and not recommended for tests without
a target triple. The issue has been benign as we recognize
nvptx{,64}-apple-darwin as ELF instead of rejecting it outrightly.
`LowerDYNAMIC_STACKALLOC()` emits the `dynamic_stackalloc` chain operand
instead of the chain produced by the `NVPTXISD::DYNAMIC_STACKALLOC`. Fix
this behavior and don't produce an unnecessary `ISD::MERGE_VALUES`.
