[mlir][spirv] Use SingleBlock + NoTerminator for spv.module

This allows us to remove the `spv.mlir.endmodule` op and
all the code associated with it.

Along the way, tightened the APIs for `spv.module` a bit
by removing some aliases. Now we use `getRegion` to get
the only region, and `getBody` to get the region's only
block.

Reviewed By: mravishankar, hanchung

Differential Revision: https://reviews.llvm.org/D103265