Simple float4x4 passthrough function fails to compile with optimizations disabled

[ppenenko]

Description

I was investigating some validation issues in our shaders and compiled them with -Zi -O0 in an attempt to track down the offending lines of code. However, -O0 triggers additional validation issues on trivial code like this snippet. -O1 makes the issue go away.

https://godbolt.org/z/8czEq7711

Steps to Reproduce

export float4x4 mat_passthru(float4x4 mat)
{
    return mat;
}

compiled with -T lib_6_7 -O0

Actual Behavior
error: Instruction does not dominate all uses!

Environment

• DXC version: 1.7.2207
• Host Operating System: Win 10

[damyanp]

This no longer repros in recent DXC. https://godbolt.org/z/PYMxPPbo4