Add an accelerated version of F_TO_I for x86_64
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 <b186559eaedf698788888c8a2559b4efb4db10d6@intel.com>
Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

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