Erase static sizes above some threshold.

[bjacob]

Early on around GlobalOptimization, we should just rewrite large static sizes into dynamic sizes. There is no point carrying large static sizes around, and doing so before dispatch region formation will lead to far fewer dispatch functions being created, which will be a large compilation-time speedup, see #16848.

Later in compilation, we do things that require certain sizes to be constant: for instance, vectorization. But if we do the rewrite early on, these considerations shouldn't enter the picture.