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).