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vpx_dsp/get_prob: make clip_prob branchless

+ inline the function directly as there was only one consumer
(get_prob())

this is an attempt to reduce the amount of branches to workaround an amd
bug. this change is mildly faster or neutral across x86-64, arm.

http://support.amd.com/TechDocs/44739_12h_Rev_Gd.pdf
665 Integer Divide Instruction May Cause Unpredictable Behavior

BUG=chromium:639712

Suggested-by: Pascal Massimino <pascal.massimino@gmail.com>
Change-Id: Ia91823aded79aab469dd68095d44300e8df04ed2

--- a/vpx_dsp/prob.h

+++ b/vpx_dsp/prob.h

@@ -43,13 +43,14 @@

 typedef const vpx_tree_index vpx_tree[];

-static INLINE vpx_prob clip_prob(int p) {

-  return (p > 255) ? 255 : (p < 1) ? 1 : p;

-}

-

 static INLINE vpx_prob get_prob(unsigned int num, unsigned int den) {

   if (den == 0) return 128u;

-  return clip_prob((int)(((int64_t)num * 256 + (den >> 1)) / den));

+  {

+    const int p = (int)(((int64_t)num * 256 + (den >> 1)) / den);

+    // (p > 255) ? 255 : (p < 1) ? 1 : p;

+    const int clipped_prob = p | ((255 - p) >> 23) | (p == 0);

+    return (vpx_prob)clipped_prob;

+  }

 }

 static INLINE vpx_prob get_binary_prob(unsigned int n0, unsigned int n1) {