[SPIR-V] Some variable are decorated with both Flat and PerVertexKHR

[s-perron]

Description

We are incorrectly decorating a single variable with both flat and PerVertexKHR, which I believe is incorrect.

Steps to Reproduce

https://godbolt.org/z/E3djqvEzG

Actual Behavior

A single variable has contradictory decoration.

Environment

• DXC version Trunk
• Host Operating System All