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       Tidy up the code generator
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The code generation for 'case' expressions had grown
huge and gnarly.  This commit removes about 120 lines of
code, and makes it a lot easier to read too. I think the code
generated is identical.

Part of this was to simplify the StgCase data type, so
that it is more like the Core case: there is a simple list
of alternatives, and the DEFAULT (if present) must be the
first.  This tidies and simplifies other Stg passes.