[mlir][spirv] Replace SPIRVOpLowering with OpConversionPattern

The dialect conversion framework was enhanced to handle type
conversion automatically. OpConversionPattern already contains
a pointer to the TypeConverter. There is no need to duplicate it
in a separate subclass. This removes the only reason for a
SPIRVOpLowering subclass. It adapts to use core infrastructure
and simplifies the code.

Also added a utility function to OpConversionPattern for getting
TypeConverter as a certain subclass.

Reviewed By: hanchung

Differential Revision: https://reviews.llvm.org/D94080