[committed] Fix m68k bootstrap segfault with late-combine

So the m68k port has failed to bootstrap since the introduction of
late-combine.  My suspicion has been this is a backend problem.  Sure enough
after bisecting things down (thank goodness for the debug counter!) I'm happy
to report m68k (after this patch) has moved into its stage3 build for the first
time in a month.

Basically late-combine propagated an address calculation to its use points,
generating this insn (dwarf2out.c, I forget what function):

> (insn 653 652 655 (parallel [
>             (set (mem/j:DI (plus:SI (plus:SI (reg/f:SI 9 %a1 [orig:64 _67 ] [64])
>                             (reg:SI 0 %d0 [321]))
>                         (const_int 20 [0x14])) [0 slot_204->dw_attr_val.v.val_unsigned+0 S8 A16])
>                 (sign_extend:DI (mem/c:SI (plus:SI (reg/f:SI 14 %a6)
>                             (const_int -28 [0xffffffffffffffe4])) [870 %sfp+-28 S4 A16])))
>             (clobber (reg:SI 0 %d0))
>         ]) "../../../gcc/gcc/dwarf2out.cc":24961:23 93 {extendsidi2}
>      (expr_list:REG_DEAD (reg/f:SI 9 %a1 [orig:64 _67 ] [64])
>         (expr_list:REG_DEAD (reg:SI 0 %d0 [321])
>             (expr_list:REG_UNUSED (reg:SI 0 %d0)
>                 (nil)))))
Note how the output uses d0 in the address calculation and the clobber uses d0.

It matches this insn in the md file:

> (define_insn "extendsidi2"
>   [(set (match_operand:DI 0 "nonimmediate_operand" "=d,o,o,<")
>         (sign_extend:DI
>          (match_operand:SI 1 "nonimmediate_src_operand" "rm,rm,r<Q>,rm")))
>    (clobber (match_scratch:SI 2 "=X,&d,&d,&d"))]
>   ""
> {
>   if (which_alternative == 0)
>     /* Handle alternative 0.  */
>     {
>       if (TARGET_68020 || TARGET_COLDFIRE)
>         return "move%.l %1,%R0\;smi %0\;extb%.l %0";
>       else
>         return "move%.l %1,%R0\;smi %0\;ext%.w %0\;ext%.l %0";
>     }
>
>   /* Handle alternatives 1, 2 and 3.  We don't need to adjust address by 4
>      in alternative 3 because autodecrement will do that for us.  */
>   operands[3] = adjust_address (operands[0], SImode,
>                                 which_alternative == 3 ? 0 : 4);
>   operands[0] = adjust_address (operands[0], SImode, 0);
>
>   if (TARGET_68020 || TARGET_COLDFIRE)
>     return "move%.l %1,%3\;smi %2\;extb%.l %2\;move%.l %2,%0";
>   else
>     return "move%.l %1,%3\;smi %2\;ext%.w %2\;ext%.l %2\;move%.l %2,%0";
> }
>   [(set_attr "ok_for_coldfire" "yes,no,yes,yes")])
Note the smi/ext instruction pair in the case for alternatives 1..3.  Those
clobber the scratch register before we're done consuming inputs.  The scratch
register really needs to be marked as an earlyclobber.

That fixes the bootstrap problem, but a cursory review of m68k.md is not
encouraging.  I will not be surprised at all if there's more of this kind of
problem lurking.

But happy to at least have m68k bootstrapping again.   It's failing the
comparison test, but definitely progress.

	* config/m68k/m68k.md (extendsidi2): Add missing early clobbers.

gcc/config/m68k/m68k.md

@@ -1887,7 +1887,7 @@
   [(set (match_operand:DI 0 "nonimmediate_operand" "=d,o,o,<")
 	(sign_extend:DI
 	 (match_operand:SI 1 "nonimmediate_src_operand" "rm,rm,r<Q>,rm")))
-   (clobber (match_scratch:SI 2 "=X,d,d,d"))]
+   (clobber (match_scratch:SI 2 "=X,&d,&d,&d"))]
   ""
 {
   if (which_alternative == 0)