x86: Improve SSSE3 SGR asm

 * Use the same approach as AVX2 of using floating-point reciprocal
   instructions to replace dav1d_sgr_x_by_x[] table lookups.

 * Optimize clipping of p-values in the 10bpc code.

 * Rename some macros to clarify their functionality.

 * Implement various minor tweaks.