sys_core_fold: Eliminate warnings for unused terms

The optimization introduced in 0a0d39d351fc would cause spurious
warnings of the type: "a term is constructed, but never used".

To avoid the warning, we must mark not only tuples and lists as
compiler_generated, but also each element. We must also propagate
compiler_generated annotations in lets. For example, if we have:

  let <X -| ['compiler_generated']> = 42 in X + 1

we must propagate the compiler_generated annotation to the literal
when do constant propagation:

  42 -| ['compiler_generated'] + 1

1 files changed, 8 insertions, 0 deletions

diff --git a/lib/compiler/test/warnings_SUITE.erl b/lib/compiler/test/warnings_SUITE.erl
index f6ba75577d..471ded219f 100644
--- a/lib/compiler/test/warnings_SUITE.erl
+++ b/lib/compiler/test/warnings_SUITE.erl
@@ -739,6 +739,14 @@ no_warnings(Config) when is_list(Config) ->
                 case R0 of
 	          {r,V1,_V2,V3} -> {r,V1,\"def\",V3}
                 end.
+
+              d(In0, Bool) ->
+                {In1,Int} = case id(Bool) of
+                              false -> {In0,0}
+                            end,
+                [In1,Int].
+
+              id(I) -> I.
            ">>,
            [],
            []}],