operator[] on a pointer was using the vector register size (16 bytes)
instead of the size of the (emulated) vector type.

Change-Id: Ice8677b41a3af81176fe9495fd02ede9f8b0482f
Reviewed-on: https://swiftshader-review.googlesource.com/8569Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>