Update structured-to-memref pass to support the new pass pipeline

[nhat-nguyen]

This PR simplifies the structured-to-memref pass responsible for converting structured triton load / store ops to memref load / store ops. This is part of the work to allow triton-shared to lower gather / scatter pointer sequences. Previously, this pass is also responsible for converting scalar pointer load and store into memref; that transformation has now been moved to unstructured-to-memref.

In addition, the PR also updates the triton-to-linalg-experimental pass to fully utilize all the new passes. Once merged, triton-shared now fully supports gather / scatter. An example test (test_gather_scatter.py) is also added to demonstrate this new capability.

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Intended lowering pipeline:

• triton-to-structured (no changes):
  • analyzes structured addptr sequences
    • introduces tts.make_tptr %ptr_arg with offsets and strides
    • introduces tts.load and tts.store
  • leaves unstructured addptr sequences and their corresponding tt.load and tt.store intact
• fold-unstructured-ptr (Introduce fold-unstructured-ptr pass #210):
  • converts all unstructured addptr sequences into sequences that compute pointer offsets
    • introduces tts.make_unstructured_tptr %ptr_arg %offsets
  • removes all tt.addptr
• structured-to-memref (this PR):
  • currently converts everything to memref including scalar addptr and kernel arguments
  • will change to just convert ops in the tts dialect to memref with the exception of tts.make_unstructured_tptr
• unstructured-to-memref (Introduce unstructured-to-memref pass #216 ):
  • converts the remaining unstructured tt.load, tt.store, and tts.make_unstructured_tptr into memref
• triton-ptr-to-memref (Introduce triton-ptr-to-memref pass #211):
  • converts kernel arguments with pointer type to memref