[hlsl][ir] Rename expected files to drop .ir.

This renames all of the expected files to remove the .ir naming and
updates main.go to generate without .ir.

Bug: 421916945
Change-Id: I1262fdff9fd27961bb662b5a3f1fa176b26320a5
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/248974
Reviewed-by: James Price <jrprice@google.com>
Auto-Submit: dan sinclair <dsinclair@chromium.org>