mmx: Use _mm_mulhi_pu16

mmx: Use _mm_mulhi_pu16

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

This instruction is already used in the SSE2 code.

Reduces code size by ~5%.

amd64
  text    data     bss     dec     hex filename
 31325    2240       0   33565    831d .libs/libpixman_mmx_la-pixman-mmx.o
 29925    2240       0   32165    7da5 .libs/libpixman_mmx_la-pixman-mmx.o

x86
  text    data     bss     dec     hex filename
 29165    1792       0   30957    78ed .libs/libpixman_mmx_la-pixman-mmx.o
 27677    1792       0   29469    731d .libs/libpixman_mmx_la-pixman-mmx.o

arm
  text    data     bss     dec     hex filename
 31632    1792       0   33424    8290 .libs/libpixman_iwmmxt_la-pixman-mmx.o
 30176    1792       0   31968    7ce0 .libs/libpixman_iwmmxt_la-pixman-mmx.o

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