[mlir][SPIRV] Split up StandardToSPIRV now that the Standard dialect is gone

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