Fix sign masks of narrow vectors.

Fix sign masks of narrow vectors.

The SignMask intrinsic returns the most significant bit of each byte in
a 128-bit vector. For 'emulated' 8-byte vectors we expect the upper
half to be ignored.

Change-Id: I994e35fb9c21710c35a89add42297c3c9bad07a4
Reviewed-on: https://swiftshader-review.googlesource.com/10988
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>