[llvm] [SPIR-V] Fix ExecutionMode generation (PR #143888)

[Nathan Gauër via llvm-commits]

Keenuts wrote:

> Can you add a test for this?

This requires a fragment shader with an input. Found it while building `vk::location`. For now we have no input on FS which builds correctly.
Once we have a test using `vk::location` with FS, this will be covered

https://github.com/llvm/llvm-project/pull/143888