ARMv6: Add fast path for over_reverse_n_8888

ARMv6: Add fast path for over_reverse_n_8888

Benchmark results, "before" is upstream commit
c343846 lowlevel-blt-bench: add in_reverse_8888_8888 test
and "after" is with this patch only added on top.

lowlevel-blt-bench, over_reverse_n_8888, 100 iterations:

       Before          After
      Mean StdDev     Mean StdDev   Confidence   Change
L1    15.1    0.1    274.5    2.3    100.00%   +1718.9%
L2    12.8    0.3    181.8    0.7    100.00%   +1315.5%
M     10.8    0.0     77.9    0.0    100.00%    +621.2%
HT     9.7    0.0     29.4    0.2    100.00%    +204.9%
VT     9.5    0.0     26.7    0.1    100.00%    +179.3%
R      9.3    0.0     25.3    0.1    100.00%    +173.6%
RT     6.0    0.1     11.0    0.2    100.00%     +82.9%

At most 16 outliers rejected per case per set.

cairo-perf-trace with trimmed traces, 30 iterations:

                                    Before          After
                                   Mean StdDev     Mean StdDev   Confidence   Change
t-poppler.trace                    12.9    0.1      9.7    0.0    100.00%     +32.6%
t-firefox-talos-gfx.trace          33.2    0.7     32.9    0.4     95.23%      +0.9%  (insignificant)
t-firefox-particles.trace          27.4    0.1     27.3    0.2     99.65%      +0.4%
t-firefox-canvas-alpha.trace       20.5    0.3     20.5    0.3     57.51%      +0.3%  (insignificant)
t-poppler-reseau.trace             22.4    0.1     22.4    0.1     95.69%      +0.3%  (insignificant)
t-firefox-fishtank.trace           13.2    0.0     13.2    0.0     99.84%      +0.1%
t-swfdec-giant-steps.trace         14.9    0.0     14.9    0.0     87.68%      +0.1%  (insignificant)
t-swfdec-youtube.trace              7.8    0.0      7.8    0.0     35.22%      +0.1%  (insignificant)
t-firefox-planet-gnome.trace       11.5    0.0     11.5    0.0     29.37%      +0.0%  (insignificant)
t-firefox-fishbowl.trace           21.2    0.0     21.2    0.0     18.09%      +0.0%  (insignificant)
t-grads-heat-map.trace              4.4    0.0      4.4    0.0      1.84%      +0.0%  (insignificant)
t-firefox-paintball.trace          18.0    0.0     18.0    0.0     33.43%      -0.0%  (insignificant)
t-firefox-talos-svg.trace          20.5    0.0     20.5    0.1     68.56%      -0.1%  (insignificant)
t-midori-zoomed.trace               8.0    0.0      8.0    0.0     99.98%      -0.1%
t-firefox-canvas-swscroll.trace    32.1    0.1     32.1    0.1     85.27%      -0.1%  (insignificant)
t-gnome-system-monitor.trace       17.2    0.0     17.2    0.0     99.97%      -0.2%
t-firefox-chalkboard.trace         36.5    0.0     36.6    0.0    100.00%      -0.2%
t-firefox-asteroids.trace          11.1    0.0     11.1    0.0    100.00%      -0.2%
t-firefox-canvas.trace             17.9    0.0     18.0    0.0    100.00%      -0.3%
t-chromium-tabs.trace               4.9    0.0      4.9    0.0     97.95%      -0.3%  (insignificant)
t-xfce4-terminal-a1.trace           4.8    0.0      4.8    0.0    100.00%      -0.4%
t-firefox-scrolling.trace          31.1    0.1     31.2    0.1    100.00%      -0.5%
t-evolution.trace                  13.7    0.1     13.8    0.1     99.99%      -0.6%
t-gnome-terminal-vim.trace         22.0    0.2     22.2    0.1     99.99%      -0.7%
t-gvim.trace                       33.2    0.2     33.5    0.2    100.00%      -0.8%

At most 6 outliers rejected per case per set.

Cairo perf reports the running time, but the change is computed for
operations per second instead (inverse of running time).

Changes in the order of +/- 1% can be accounted for measurement errors,
even if they are deemed to be statistically significant. This claim is
based on comparing two 30-iteration identical "before" runs using the
exact same binaries, and observing changes from -0.4% to +0.5% with
>=99% confidence.

Confidence is based on Welch's t-test.

v4, Pekka Paalanen <pekka.paalanen@collabora.co.uk> :
	Rebased, re-benchmarked on Raspberry Pi, commit message.