[testsuite] Fix guality/pr45882.c for flto

[testsuite] Fix guality/pr45882.c for flto

Atm this test in pr45882.c fails:
...
  int d = a[i];  /* { dg-final { gdb-test 16 "d" "112" } } */
...
as follows:
...
FAIL: gcc.dg/guality/pr45882.c   -O2 -flto -fuse-linker-plugin \
      -fno-fat-lto-objects  line 16 d == 112
...

In more detail, gdb fails to print the value of d:
...
Breakpoint 1, foo (i=i@entry=7, j=j@entry=7) at pr45882.c:16
16        ++v;
$1 = <optimized out>
$2 = 112
<optimized out> != 112
...

Variable d is a local variable in function foo, initialized from global array a.
When compiling, first cddce1 removes the initialization of d in foo, given
that d is not used afterwards.  Then ipa marks array a as write-only, and
removes the stores to array a in main.  This invalidates the location
expression for d, which points to a[i], so it is removed, which is why gdb
ends up printing <optimized out> for d.

This patches fixes the fail by adding attribute used to array a, preventing
array a from being marked as write-only.

Tested on x86_64.

2018-06-21  Tom de Vries  <tdevries@suse.de>

	* gcc.dg/guality/pr45882.c (a): Add used attribute.

From-SVN: r261845