[mlir][Vector][NFC] Small vector masking clean-up

We stored static (int) and dynamic (Value) iteration space dims separately
and then merged them by creating constant ops for the static ones. This
merge happened multiple times during vectorization. This PR changes that
to perform the merge once and store in the state instead of the dynamic
values in isolation.

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D147351