Fix one more `-Wswitch` warning. NFC.
Fix one more `-Wswitch` warning. NFC.

I missed this in https://github.com/GPUOpen-Drivers/llpc/pull/1501.

GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc