Fix an old but subtle bug in the Simplifier

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!