Also disable the assignment operator implicitly doing a broadcast.
Instead the explicit constructor must be used. This prevents bugs like
assigning a scalar to one element, while it was intended to broadcast
(especially when using [] on an array of vectors).

Bug: b/146224130
Bug: b/144825072
Change-Id: I43c977e256e134f6707b5c19252ded08db73b270
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39630
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>