This commit marks the type converter in `populate...` functions as
`const`. This is useful for debugging.
Patterns already take a `const` type converter. However, some
`populate...` functions do not only add new patterns, but also add
additional type conversion rules. That makes it difficult to find the
place where a type conversion was added in the code base. With this
change, all `populate...` functions that only populate pattern now have
a `const` type converter. Programmers can then conclude from the
function signature that these functions do not register any new type
conversion rules.
Also some minor cleanups around the 1:N dialect conversion
infrastructure, which did not always pass the type converter as a
`const` object internally.
StandardToSPIRV currently contains an assortment of patterns converting from
different dialects to SPIRV. This commit splits up StandardToSPIRV into separate
conversions for each of the dialects involved (some of which already exist).
Differential Revision: https://reviews.llvm.org/D120767
