Re-land [EmitC] Add common baseclass for conversions and simplify pat…

…terns. (iree-org#15267)

This reverts iree-org#15265 in order to
re-land iree-org#14717.

Fix was found by searching the repo for similar code and finding
https://github.com/openxla/iree/blob/284c21005f62febb64bf8b61aec13a8cf7c9e346/compiler/src/iree/compiler/InputConversion/StableHLO/StableHLOToArith.cpp#L21-L25

ci-extra: build_test_all_windows