How valuable is that? If it's significant, we may want to consider converting TPF to SSA form as well in the SPIR-V backend.
It seems to me quite likely that translating sm4 to SSA would be more expensive than passing it to the driver (and letting the driver translate it to SSA internally, probably), but translating sm6 to temps would be more expensive than passing it through to the driver. I don't think we have any special knowledge that would let us convert sm4 to SSA more efficiently. Granted, maybe those differences aren't significant, but I hesitate to just dismiss them out of hand.
Also: if we are going to translate HLSL to vsir, and then perform optimizations, or raising passes, on that vsir, then it is distinctly easier to have those in SSA form, then convert the SSA form to registers once the actual instruction sequence has been finalized.
I haven't tried it, nor actually read Conor's code, but it doesn't seem likely that adding SSA is going to complexify the code that much. [Tracking def-use chains would, but I don't think we need that...]