Emit shorter code for strict bitstring generators

The skip clause of the generator (which handles the case when the
bitstring does not match the pattern) used to test whether the
bit_size of the argument is above 0, which requires two BIF calls.

The new version pattern matches against any binary and checks that it
is not the empty binary with a single BIF call.

lib/compiler/src/v3_core.erl

@@ -1965,38 +1965,18 @@ generator(Line, {Generate,Lg,P,E}, Gs, St0) when Generate =:= b_generate;
             {AccSegs,Tail,TailSeg,St2} = append_tail_segment(Segs, St1),
             AccPat = Cp#ibinary{segments=AccSegs},
             {Cg,St3} = lc_guard_tests(Gs, St2),
-            {[BitStr,BitSize],St4} = new_vars(2, St3),
-            %% The non-empty bistring guard could be implemented either
-            %% as
-            %%
-            %% bit_size(BitStr) > 0
-            %%
-            %% or as
-            %%
-            %% is_bitstring(BitStr), BitStr =/= <<>>
-            %%
-            %% However, in the second case the compiler incorrectly
-            %% decides the is_bitstring/1 call had no effect and optimises
-            %% it away. So we stick to the first option instead.
-            BitSizeCall = #icall{anno=GAnno,
-                                 module=#c_literal{anno=GA,val=erlang},
-                                 name=#c_literal{anno=GA,val=bit_size},
-                                 args=[BitStr]},
-            CompCall = #icall{anno=GAnno,
-                              module=#c_literal{anno=GA,val=erlang},
-                              name=#c_literal{anno=GA,val='>'},
-                              args=[BitSize,#c_literal{anno=GA,val=0}]},
-            Guard = #iprotect{anno=GAnno,
-                              body=[#iset{anno=GAnno,var=BitSize,arg=BitSizeCall},
-                                    CompCall]},
-            {Ce,Pre,St5} = safe(E, St4),
+            Guard = #icall{anno=GAnno,
+                           module=#c_literal{anno=GA,val=erlang},
+                           name=#c_literal{anno=GA,val='=/='},
+                           args=[Tail,#c_literal{anno=GA,val= <<>>}]},
+            {Ce,Pre,St4} = safe(E, St3),
             Gen = #igen{anno=GAnno,acc_pat=AccPat,acc_guard=Cg,
-                        nomatch_pat=BitStr,
+                        nomatch_pat=#ibinary{anno=#a{anno=LA},segments=[TailSeg]},
                         nomatch_guard=[Guard],
-                        nomatch_mode=BitStr,
+                        nomatch_mode=Tail,
                         tail=Tail,tail_pat=#ibinary{anno=#a{anno=LA},segments=[TailSeg]},
                         arg={Pre,Ce}},
-            {Gen,St5}
+            {Gen,St4}
     catch
         throw:nomatch ->
             {Ce,Pre,St1} = safe(E, St0),