aarch64: Fix another bug in aarch64_add_offset_1 [PR94121]

aarch64: Fix another bug in aarch64_add_offset_1 [PR94121]

> I'm getting this ICE with -mabi=ilp32:
>
> during RTL pass: fwprop1
> /opt/gcc/gcc-20200312/gcc/testsuite/gcc.dg/pr94121.c: In function 'bar':
> /opt/gcc/gcc-20200312/gcc/testsuite/gcc.dg/pr94121.c:16:1: internal compiler error: in decompose, at rtl.h:2279

That is a preexisting issue, caused by another bug in the same function.
When mode is SImode and moffset is 0x80000000 (or anything else with the
bit 31 set), we need to sign-extend it.

2020-03-13  Jakub Jelinek  <jakub@redhat.com>

	PR target/94121
	* config/aarch64/aarch64.c (aarch64_add_offset_1): Use gen_int_mode
	instead of GEN_INT.

(cherry picked from commit c2f836c413b1e9ae45598338b4a2ecd33bd926fb)