[MLIR][SPIRVToLLVM] SPIRV function fix and nits

This patch addresses the comments from https://reviews.llvm.org/D83030 and
https://reviews.llvm.org/D82639. `this->` is removed when not inside the
template. Also, type conversion for `spv.func` takes `convertRegionTypes()`
in order to apply type conversion on all blocks within the function.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D83786