Fix #3170: Allow coalescing of the same node during register allocation.

  The x86 code generator sometimes generates nop moves like
  movl %vI_a %vI_a, but we'll just ignore them for coalescing
  purposes instead of emitting an error. It shouldn't hurt anything.

compiler/utils/GraphOps.hs

@@ -335,6 +335,9 @@ coalesceNodes aggressive triv graph (k1, k2)
 	, not $ elementOfUniqSet kMin (nodeConflicts nMax)
 	, not $ elementOfUniqSet kMax (nodeConflicts nMin)
 
+	-- can't coalesce the same node
+	, nodeId nMin /= nodeId nMax
+
 	= coalesceNodes_merge aggressive triv graph kMin kMax nMin nMax
 
 	-- don't do the coalescing after all
@@ -350,9 +353,6 @@ coalesceNodes_merge aggressive triv graph kMin kMax nMin nMax
 	| not (isNothing (nodeColor nMin) && isNothing (nodeColor nMax))
 	= error "GraphOps.coalesceNodes: can't coalesce colored nodes."
 
-	| nodeId nMin == nodeId nMax
-	= error "GraphOps.coalesceNodes: can't coalesce the same node."
-
 	---
 	| otherwise
 	= let