Fix ICE w/ mingw[1][2][3] when using -fno-omit-frame-pointer.

Patch is from the proposed fix in [4].

[1] https://bugs.gentoo.org/932319
[2] https://github.com/msys2/MINGW-packages/issues/20861
[3] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115038
[4] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115038#c4
--- a/gcc/gcc/fold-mem-offsets.cc
+++ b/gcc/gcc/fold-mem-offsets.cc
@@ -492,5 +492,5 @@
   rtx_insn *def = get_single_def_in_bb (insn, reg);
 
-  if (!def || GET_CODE (PATTERN (def)) != SET)
+  if (!def || RTX_FRAME_RELATED_P (def) || GET_CODE (PATTERN (def)) != SET)
     return 0;