Emulated vectors have fewer elements than their underlying type, e.g.
v4i16 is stored in the lower half of a v8i16 vector, but for constant
vector construction we need to provide values for all the elements of
the underlying vector.

Bug b/65124545

Change-Id: Iaf942c4d517b1d4d464894ec501f7d339ae86438
Reviewed-on: https://swiftshader-review.googlesource.com/11908Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>