[HW] Fold extracts of constant structs

[fzi-hielscher]

Fold hw.struct_extract operations of hw.aggregate_constant to the constant value of their respective field.

This breaks a test in HWtoLLVM, which checks correct conversion of the extract(constant) pattern. Since dialect conversion inevitably performs a fold on the input operations before converting, the extract operation will always end up as hw.constant. There does not appear to be a dedicated test for converting constants, so I opted to just keep it in and adapt the expected output.

[uenoku]

LGTM

[fzi-hielscher]

Merging, assuming there are no objections to my tampering with the HWtoLLVM test, @fabianschuiki @maerhart ?

[maerhart]

Thanks for implementing this folder @fzi-hielscher!
Regarding the HWToLLVM test-case: this one is supposed to test the aggregate_constant operation, I'm not quite sure anymore why there's also an extract in there (probably that it isn't completely folded away). So, as long as the addressof and load are still there, it's all good.

[fabianschuiki]

Thanks a lot @fzi-hielscher 🎉 !