Widen acceptance window of T12545 (#19414)

This test flip-flops by +-1% in arbitrary changes in CI.
While playing around with `-dunique-increment`, I could reproduce
variations of 3% in compiler allocations, so I set the acceptance window
accordingly.

Fixes #19414.

testsuite/tests/perf/compiler/all.T

@@ -211,9 +211,12 @@ test('T12234',
      compile,
      [''])
 
+# T12545 is sensitive to -dunique-increments changes, see #19414.
+# I've seen variations of 3% by playing with that parameter, so that's the
+# current acceptance window.
 test('T12545',
      [ only_ways(['normal']),
-       collect_compiler_stats('bytes allocated', 1),
+       collect_compiler_stats('bytes allocated', 3),
        extra_clean(['T12545a.hi', 'T12545a.o'])
      ],
      multimod_compile,