[dev.ssa] cmd/compile/internal/ssa: distinguish exit and return blocks

It is confusing to have exceptional edges jump back into
real code.  Distinguish return blocks, which execute acutal
code, and the exit block, which is a merge point for the regular
and exceptional return flow.

Prevent critical edge insertion from adding blocks on edges
into the exit block.  These added blocks serve no purpose and
add a bunch of dead jumps to the assembly output.  Furthermore,
live variable analysis is confused by these jumps.

Change-Id: Ifd69e6c00e90338ed147e7cb351b5100dc0364df
Reviewed-on: https://go-review.googlesource.com/14254
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/ssa/gen/genericOps.go

@@ -375,6 +375,7 @@ var genericBlocks = []blockData{
 	{name: "If"},    // 2 successors, if control goto Succs[0] else goto Succs[1]
 	{name: "Call"},  // 2 successors, normal return and panic
 	{name: "First"}, // 2 successors, always takes the first one (second is dead)
+	{name: "Ret"},   // 1 successor, branches to exit
 }
 
 func init() {