Fix SEGV when dumping MIR CFG

I turned on kDebugDumpCFG to try to dump the CFGs, and I found
that they caused a segmentation violation.  The problem was that
the orig_sreg was -1, and IsConst then blew up.

Change-Id: Ib9acda0b11ce486e878ef2ccfae3b1bc1f82fdfb
Signed-off-by: Mark Mendell <mark.p.mendell@intel.com>
1 file changed