[SPIR-V] Varying error behavior with bit functions on 16 bit types

Description

When using functions like firstbithigh, firstbitlow, and countbits, there are varying errors. The correct error is something along the lines of error: firstbithigh is currently limited to 32-bit width components when targetting SPIR-V however that isn't always the result.

Steps to Reproduce

Example of "correct" behavior:
https://godbolt.org/z/ecPqhxWGx
Example of segfault:
https://godbolt.org/z/4qYG1Y8Mv
Example of possibly correct, but more vague error:
https://godbolt.org/z/5x37qdnxs

Actual Behavior

I feel the correct behavior is, if asking for these ops to work for 16 bit values is off the table, for them to all produce the same error (and definitely not segfault)

Environment

• DXC version 1.8.2407
• Host Operating System - Windows version10.0.22621 Build 22621 (and also some version of ubuntu 22 but I don't know exactly)

Reproduce the three linked Godbolt cases with DXC targeting SPIR-V, comparing firstbithigh, firstbitlow, and countbits on 16-bit types. Trace the diagnostics and the segfaulting case through the compiler's SPIR-V path; done means these operations consistently produce the intended unsupported-width error without crashing.