[Arc] Keep just one parameter if it's given multiple times

[elhewaty]

This pull request adds a new canonical form for the arc.vectorize operation.
Given the following IR

%1:4 = arc.vectorize(%0#0, %0#1, %0#2, %0#3), (%a, %b, %c, %d), (%0#0, %0#1, %0#2, %0#3)

We can remove the repeated input to simplify it as:

%1:4 = arc.vectorize(%0#0, %0#1, %0#2, %0#3), (%a, %b, %c, %d)

This can reduce the time of the unpacking of vector elements.

[elhewaty]

This can be generalized to remove any repeated pattern in the input, what do you think?
If the input patterns are scalars that won't be useful (I think), but if they are vectors maybe we can save more extraction time?

[maerhart]

Yes, I think this is a useful canonicalization regardless of how much you can save in vector packing. 👍
It can reduce the use-count of values and thus enable more applications of your other canonicalization pattern, for example.

[fabianschuiki]

This is a very useful canonicalization! LGTM, besides a minor comment about SmallVector maybe not being necessary.

[fabianschuiki]

LGTM!