Simplify the classes that support SPIR-V conversion.

Modify the Type converters to have a SPIRVBasicTypeConverter which
only handles conversion from standard types to SPIRV types. Rename
SPIRVEntryFnConverter to SPIRVTypeConverter. This contains the
SPIRVBasicTypeConverter within it.

Remove SPIRVFnLowering class and have separate utility methods to
lower a function as entry function or a non-entry function. The
current setup could end with diamond inheritence that is not very
friendly to use.  For example, you could define the following Op
conversion methods that lower from a dialect "Foo" which resuls in
diamond inheritance.

template<typename OpTy>
class FooDialect : public SPIRVOpLowering<OpTy> {...};
class FooFnLowering : public FooDialect, SPIRVFnLowering {...};

PiperOrigin-RevId: 263597101