This CL enables subzero to lower shufflevector instructions in x86 using
pshufb (only for SSE 4.1)

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4136
BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4077
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1917863004 .