compiler.cfg.stacks.finalize: only insert blocks if they aren't empty

Previously, a lot of empty blocks were added too.

basis/compiler/cfg/stacks/finalize/finalize-docs.factor

@@ -1,4 +1,5 @@
-USING: assocs compiler.cfg help.markup help.syntax ;
+USING: assocs compiler.cfg compiler.cfg.instructions help.markup
+help.syntax ;
 IN: compiler.cfg.stacks.finalize
 
 HELP: inserting-peeks
@@ -24,7 +25,7 @@ HELP: finalize-stack-shuffling
 
 HELP: visit-edge
 { $values { "from" basic-block } { "to" basic-block } }
-{ $description "If both blocks are subroutine calls, don't bother computing anything." } ;
+{ $description "If required, insert a block containing " { $link ##peek } " and " { $link ##replace } " instructions on the edge between the 'from' and 'to' block. If no such instructions are needed, then no block is inserted." } ;
 
 ARTICLE: "compiler.cfg.stacks.finalize" "Stack finalization"
 "This pass inserts peeks and replaces." ;

basis/compiler/cfg/stacks/finalize/finalize-tests.factor

@@ -5,10 +5,9 @@ compiler.cfg.utilities kernel sequences tools.test ;
 IN: compiler.cfg.stacks.finalize.tests
 
 {
-    T{ ##branch f f }
-    T{ ##branch f f }
+    "there" "hi"
 } [
-    V{ } clone 1 insns>block V{ } clone 2 insns>block
+    V{ "hi" } clone 1 insns>block V{ "there" } clone 2 insns>block
     2dup connect-bbs 2dup visit-edge
     [ successors>> first instructions>> first ]
     [ predecessors>> first instructions>> first ] bi*

basis/compiler/cfg/stacks/finalize/finalize.factor

@@ -37,10 +37,10 @@ ERROR: bad-peek dst loc ;
     [ dup n>> 0 < [ 2drop ] [ ##replace, ] if ] each-insertion ;
 
 : visit-edge ( from to -- )
-    2dup [ kill-block?>> ] both? [ 2drop ] [
-        2dup [ [ insert-replaces ] [ insert-peeks ] 2bi ##branch, ] V{ } make
+    2dup [ [ insert-replaces ] [ insert-peeks ] 2bi ##branch, ] V{ } make
+    dup length 1 > [
         insert-basic-block
-    ] if ;
+    ] [ 3drop ] if ;
 
 : visit-block ( bb -- )
     [ predecessors>> ] keep '[ _ visit-edge ] each ;