[mlir][dataflow] Remove Abstract{Sparse,Dense}Lattice::isAtFixpoint() and an ineffect...

[mlir][dataflow] Remove Abstract{Sparse,Dense}Lattice::isAtFixpoint() and an ineffective optimization to simplify public API

Currently, in the MLIR `{Sparse,Dense}DataFlowAnalysis` API, there is a small optimization:

Before running a transfer function, if the "out state" is already at the pessimistic fixpoint (bottom lattice value), then we know that it cannot possibly be changed, therefore we can skip the transfer function.

I benchmarked and found that this optimization is ineffective, so we can remove it and simplify `{Sparse,Dense}DataFlowAnalysis`. In a subsequent patch, I plan to change/remove the concept of the pessimistic fixpoint so that the API is further simplified.

Benchmark: I ran the following tests 5 times (after 3 warmup runs), and timed the `initializeAndRun()` function.

| Test | Before (us) | After (us) |
| mlir-opt -test-dead-code-analysis mlir/test/Analysis/DataFlow/test-dead-code-analysis.mlir | 181.2536 | 187.7074 |
| mlir-opt -- -test-dead-code-analysis mlir/test/Analysis/DataFlow/test-last-modified-callgraph.mlir | 109.5504 | 105.0654 |
| mlir-opt -- -test-dead-code-analysis mlir/test/Analysis/DataFlow/test-last-modified.mlir | 333.3646 | 322.4224 |
| mlir-opt -- -allow-unregistered-dialect -sccp mlir/test/Analysis/DataFlow/test-combined-sccp.mlir | 1027.1492 | 1081.818 |

Note: `test-combined-sccp.mlir` is crafted by combining `mlir/test/Transforms/sccp.mlir`, `mlir/test/Transforms/sccp-structured.mlir` and `mlir/test/Transforms/sccp-callgraph.mlir`.

Reviewed By: aartbik, Mogball

Differential Revision: https://reviews.llvm.org/D131660