Add array slice update IR node

[meheff]

For array types in the IR we the following ops:

• array_index : return individual element
• array_slice: return contiguous elements
• array_update : update individual element

But we don't have a way of updating contiguous elements of an array (ie, array_slice_update). This is a hole in our IR semantics.

Possible definition of new op:

result = array_update_slice(array, slice, indices=[idx_{0}, ... , idx_{N-1}], assumed_in_bounds=<true|false>)

Where:

• array: array of at least N dimensions with element type T
• slice: array of type T[K]
• index_{i}: arbitrary bits types
• result: same type as array

Returns a copy of the input array with the K elements starting at the given indices replaced with the elements of slice. Elements of the slice to update which fall out of bounds of array are ignored.