[Mir] Consider using direct computed indices for TraceAccess and PublicInputAccess

This is a followup issue of https://github.com/0xMiden/air-script/pull/444.

Currently, the Mir values for PublicInputAccess and TraceAccess only take constant index and column references.
However, they can be accessed through computed indices. As a workaround, we currently wrap them in Accessor nodes (and handle the index of the accessor as an offset for the PublicInputAccess and TraceAccess).

However, to stay consistant, we should only have Accessor nodes with Index access types to index into a vector, not as offsets for scalar values into a vector.

Instead, PublicInputAccess and TraceAccess should reference node in the graph.

Examine the Mir (Mid-level IR) representation, focusing on the PublicInputAccess and TraceAccess node types. Look at how Accessor nodes are currently used as a workaround. Understand the graph structure to modify these nodes to reference other nodes for computed indices. Review the changes from pull request #444 for context.

索引モデルが issue の本文から書いたものです。