Fix an old but subtle bug in the Simplifier
I got a Core Lint failure when compiling System.Win32.Info in the
Win32 package. It was very delicate: adding or removing a function
definition elsewhere in the module (unrelated to the error) made the
error go away.
Happily, I found it. In SimplUtils.prepareDefault I was comparing an
InId with an OutId. We were getting a spurious hit, and hence doing
a bogus CaseMerge.
This bug has been lurking ever since I re-factored the way that case
expressions were simplified, about 6 months ago!