Add a transform to truncate unused user interstage variables by
adding a new truncated shader io struct wrapper of the original
one, with a truncate function to do the assignments called
at the return statement.
This transform is meant to be run after CanonicalizeEntryPointIO,
and will only be run under hlsl/generator_impl.cc to workaround
the extra register limitation for interstage variables on D3D FXC.
Bug: dawn:1493
Change-Id: I69081189ad7d4b76f2371fcc079f67dced2e9944
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/104620
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Shrek Shao <shrekshao@google.com>
