Fix to MirGraph::GetSSANameWithConst()

Don't call ConstantValueWide() for a wide constant if high_word is set,
as it will DCHECK if you try to get the value for the high word.

Change-Id: I046ee3e6833ceb556a3c2dbc95699882d30d65f9
Signed-off-by: Mark Mendell <mark.p.mendell@intel.com>