Use partial conversion in TritonToLinalg

[nhat-nguyen]

Use partial conversion instead of full conversion in TritonToLinalg. With partial conversion, the pass can still succeed without having to fully cover the whole Triton dialect.

This gives more flexibility for users to use IR that we don't have coverage for yet. It also opens up the possibility of converting unstructured pointer accesses to block pointers instead of directly to linalg because with the full conversion, all triton ops have to be converted to something different.

I have also fixed incorrect usages of op.emitOpError and change them to rewriter.notifyMatchFailure.

Fixes #18.