[gcn] Fix build with RTL checking [PR94248]

Building (for offloading) a '--target=amdgcn-amdhsa' GCC with
'--enable-checking=yes,extra,rtl' fails:

    during RTL pass: split2
    [...]/source-gcc/libgcc/libgcc2.c: In function '__absvdi2':
    [...]/source-gcc/libgcc/libgcc2.c:271:1: internal compiler error: RTL check: expected code 'reg', have 'const_int' in rhs_regno, at rtl.h:1923
      271 | }
	  | ^
    0x565847 ???
	    [...]/source-gcc/gcc/rtl.c:881
    0x59a8a4 ???
	    [...]/source-gcc/gcc/rtl.h:1923
    0x12e3a5c ???
	    [...]/source-gcc/gcc/config/gcn/gcn.md:631
    [...]
    Makefile:501: recipe for target '_absvdi2.o' failed
    make[4]: *** [_absvdi2.o] Error 1
    make[4]: Leaving directory '[...]/build-gcc-offload-amdgcn-amdhsa/amdgcn-amdhsa/gfx900/libgcc'

	gcc/
	PR target/94248
	* config/gcn/gcn.md (*mov<mode>_insn): Use
	'reg_overlap_mentioned_p' to check for overlap.

Tested-by: Thomas Schwinge <thomas@codesourcery.com>