Use vmulq_n_u32(..., 0x01010101) to distribute alphas.

Use vmulq_n_u32(..., 0x01010101) to distribute alphas.

This seems to make alphas() faster and Load[24]Alphas() no slower.
The change is particularly noticeable on xfermodes that call alphas()
twice (on src and dst), with a 10-12% speedup.

Xfermode_Difference_aa	  29ms -> 28.4ms	0.98x
   Xfermode_DstATop_aa	27.2ms -> 26.7ms	0.98x
       Xfermode_Xor_aa	27.2ms -> 26.5ms	0.98x
      Xfermode_DstOver	23.6ms -> 22.9ms	0.97x
   Xfermode_DstOver_aa	27.8ms -> 26.8ms	0.96x
       Xfermode_DstOut	22.6ms -> 21.7ms	0.96x
  Xfermode_Multiply_aa	  30ms -> 28.5ms	0.95x
    Xfermode_DstOut_aa	26.1ms -> 24.8ms	0.95x
     Xfermode_DstIn_aa	25.4ms -> 24.1ms	0.95x
      Xfermode_DstATop	28.7ms ->   26ms	0.9x
     Xfermode_Multiply	35.5ms -> 31.3ms	0.88x
   Xfermode_Difference	31.8ms -> 27.7ms	0.87x
          Xfermode_Xor	30.1ms -> 26.1ms	0.87x
BUG=skia:

Committed: https://skia.googlesource.com/skia/+/343c7d15c831a3c3d8a6984c1a1b524b414012df

[mtklein_C, 2015-06-22 17:54:35]

mtklein@chromium.org changed reviewers:
+ msarett@google.com

[mtklein_C, 2015-06-22 17:55:08]

The CQ bit was checked by mtklein@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-06-22 17:55:31]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1203513002/20001

[msarett, 2015-06-22 17:58:55]

lgtm

Nice!  This is a clever!

[mtklein, 2015-06-22 18:00:10]

The CQ bit was unchecked by mtklein@google.com

[mtklein, 2015-06-22 18:00:10]

The CQ bit was checked by mtklein@google.com

[commit-bot: I haz the power, 2015-06-22 18:00:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1203513002/20001

[commit-bot: I haz the power, 2015-06-22 18:00:52]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/343c7d15c831a3c3d8a6984c1a1b524b414012df