mmx: Use _mm_shuffle_pi16

mmx: Use _mm_shuffle_pi16

The pshufw x86 instruction is part of Extended 3DNow! and SSE1. The
equivalent ARM wshufh instruction was available from the first iwMMXt
instrucion set.

This instruction is already used in the SSE2 code.

Reduces code size by ~9%.

amd64
  text    data     bss     dec     hex filename
 29925    2240       0   32165    7da5 .libs/libpixman_mmx_la-pixman-mmx.o
 27237    2240       0   29477    7325 .libs/libpixman_mmx_la-pixman-mmx.o

x86
  text    data     bss     dec     hex filename
 27677    1792       0   29469    731d .libs/libpixman_mmx_la-pixman-mmx.o
 24959    1792       0   26751    687f .libs/libpixman_mmx_la-pixman-mmx.o

arm
  text    data     bss     dec     hex filename
 30176    1792       0   31968    7ce0 .libs/libpixman_iwmmxt_la-pixman-mmx.o
 27384    1792       0   29176    71f8 .libs/libpixman_iwmmxt_la-pixman-mmx.o

Signed-off-by: Matt Turner <mattst88@gmail.com>