[mlir][vector] modified scatter/gather syntax, pass_thru mandatory

[mlir][vector] modified scatter/gather syntax, pass_thru mandatory

This change makes the scatter/gather syntax more consistent with
the syntax of all the other memory operations in the Vector dialect
(order of types, use of [] for index, etc.). This will make the MLIR
code easier to read. In addition, the pass_thru parameter of the
gather has been made mandatory (there is very little benefit in
using the implicit "undefined" values).

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D94352