Add an accelerated version of F_TO_I for x86_64

According to a quick micro-benchmark, this new version is 20% faster on my
Haswell laptop.

v2: Removed the XXX note about x86_64 from the comment
v3: Use an intrinsic instead of an __asm__ block.  This should give us MSVC
    support for free.
v4: Enable it for all x86_64 builds, not just with USE_X86_64_ASM

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/mesa/main/imports.h

@@ -274,10 +274,12 @@ static inline int IROUND_POS(float f)
    return (int) (f + 0.5F);
 }
 
+#ifdef __x86_64__
+#  include <xmmintrin.h>
+#endif
 
 /**
  * Convert float to int using a fast method.  The rounding mode may vary.
- * XXX We could use an x86-64/SSE2 version here.
  */
 static inline int F_TO_I(float f)
 {
@@ -292,6 +294,8 @@ static inline int F_TO_I(float f)
 	 fistp r
 	}
    return r;
+#elif defined(__x86_64__)
+   return _mm_cvt_ss2si(_mm_load_ss(&f));
 #else
    return IROUND(f);
 #endif