PR #141787 added code to emit the Fragment execution model. This
required emitting the OriginUpperLeft ExecutionMode. But this was done
by using the same codepath used for OpEntrypoint.
This has 2 issues:
- the interface variables were added to both OpEntryPoint and
OpExecutionMode.
- the existing OpExecutionMode logic was not used.
This commit fixes this, regrouping OpExecutionMode handling in one
place, and fixing bad codegen issue when interface variiables are added.
ef1cb8277a