Find an alternative to disabling vector combining

[alan-baker]

The latest changes to vector combining introduced some issues related to pointer bitcasts. Vector combining has been disabled temporarily, but it leads to an inefficiency in test/vector_insert_element.cl where instead of storing a single vector element the test instead stores the whole vector.

[rjodinchr]

following #1039, we could now reenable it. It fixes test/vector_insert_element.cl, but also brings a lot of other issues:

• some loads of vec8 are transformed into 8 loads.
• incorrect number of indexes in OpAccessChain with some non-scalar types.

At the moment it feels better to keep it disable.