Fix internal error in seh_cfa_offset with -O2 -fno-omit-frame-pointer

Fix internal error in seh_cfa_offset with -O2 -fno-omit-frame-pointer

The problem directly comes from the -ffold-mem-offsets pass messing up with
the prologue and the frame-related instructions, which is a no-no with SEH,
so the fix simply disconnects the pass in these circumstances.

gcc/
	PR rtl-optimization/115038
	* fold-mem-offsets.cc (fold_offsets): Return 0 if the defining
	instruction of the register is frame related.

gcc/testsuite/
	* g++.dg/opt/fmo1.C: New test.