DXC Validation Error: TGSM pointers must originate from an unambiguous TGSM global variable

In commit cf044cc960419c5e08cc8a5d7366bc8df760013c (from 23 days ago) I get the following error when enabling optimization;

error: validation errors
D:\local\Temp\c325567c-e0cd-4acd-95cc-d9f5a824c9ee.hlsl:152:22: error: TGSM pointers must originate from an unambiguous TGSM global variable.
note: at '%137 = phi float addrspace(3)* [ %133, %126 ], [ %166, %135 ]' in block '#25' of function 'main'.
D:\local\Temp\c325567c-e0cd-4acd-95cc-d9f5a824c9ee.hlsl:152:22: error: TGSM pointers must originate from an unambiguous TGSM global variable.
note: at '%138 = phi float addrspace(3)* [ %130, %126 ], [ %163, %135 ]' in block '#25' of function 'main'.
D:\local\Temp\c325567c-e0cd-4acd-95cc-d9f5a824c9ee.hlsl:156:21: error: TGSM pointers must originate from an unambiguous TGSM global variable.
note: at '%141 = phi float addrspace(3)* [ %166, %155 ], [ %133, %134 ]' in block '#26' of function 'main'.
D:\local\Temp\c325567c-e0cd-4acd-95cc-d9f5a824c9ee.hlsl:156:21: error: TGSM pointers must originate from an unambiguous TGSM global variable.
note: at '%142 = phi float addrspace(3)* [ %163, %155 ], [ %130, %134 ]' in block '#26' of function 'main'.
Validation failed.

This happens because I have the WaveIsFirstLane write to a groupshared float2 array. I can't publish the code, but I will send a DM on Discord.
When disabling optimization it works, so it seems like some optimization might change the generated DXIL.

Start by reproducing the optimized DXIL validation failure involving WaveIsFirstLane writing to a groupshared float2 array; the original HLSL code is not published, though the reporter offered to provide it privately. Compare optimized and non-optimized output and investigate the reported TGSM pointer phi values. Done means optimized compilation no longer produces the validation errors.