Fix case-merge bug that was breaking the HEAD
My re-org of the case-merging transformation introduced a bug, which led to incorrect code. This only showed up occasionally, but it generated incorrect code for PprC.pprCastReg in the stage-2 compiler. As a result the stage-2 compiler ran without crashing, but itself generated bogus C. For a change, this is one that Core Lint couldn't find, so the trail was a bit longer. The fix is easy (and commented).
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