[bug] Fix assign may lose precision warning & improve related logging

[bobcao3]

Does these few things:

1. Removed printing of "No DebugInfo Available". This is simply a lot of spam when printing IR. Don't print when there's nothing to print
2. Improve debug info handling for things that doesn't have source correlation, by first searching the immediate preceding statements for debug info (so the user at least know the ballpark), and when that fails prints the callable name & statement id
3. Fixed the issue within snode_writer that emits Assign may lose precision: unknown <- f32 like crazy. (The unknown type is caused by a missing type_check on the indexing expression)
4. Use our exception system to emit more helpful warnings for load-to-store forwarding, e.g.:

[W 06/23/24 19:08:39.945 11791278] TaichiWarning
File "/Users/bobcao3/taichi/python/taichi/lang/matrix_ops.py", line 281, in _matmul_helper:
                mat_z[i, j] = mat_z[i, j] + mat_x[i, k] * mat_y[k, j]
                                            ^^^^^^^^^^^
Loading variable 937 before anything is stored to it.

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