Use LLVM's legacy pass manager for now (#13426)

See #13427

Author: kripken

tools/building.py

@@ -560,6 +560,11 @@ def link_lld(args, target, external_symbol_list=None):
   for a in llvm_backend_args():
     cmd += ['-mllvm', a]
 
+  # LLVM has turned on the new pass manager by default, but it causes some code
+  # size regressions. For now, use the legacy one.
+  # https://github.com/emscripten-core/emscripten/issues/13427
+  cmd += ['--lto-legacy-pass-manager']
+
   # For relocatable output (generating an object file) we don't pass any of the
   # normal linker flags that are used when building and exectuable
   if '--relocatable' not in args and '-r' not in args:

tools/shared.py

@@ -523,6 +523,11 @@ def get_cflags(user_args):
              '-D__unix',
              '-D__unix__']
 
+  # LLVM has turned on the new pass manager by default, but it causes some code
+  # size regressions. For now, use the legacy one.
+  # https://github.com/emscripten-core/emscripten/issues/13427
+  c_opts += ['-flegacy-pass-manager']
+
   # Changes to default clang behavior
 
   # Implicit functions can cause horribly confusing function pointer type errors, see #2175