coverage: Rename basic_coverage_blocks to just graph

[Zalathar]

During coverage instrumentation, this variable always holds the current function's coverage graph, which is a simplified view of its MIR control-flow graph. The new name is clearer in context, and also shorter.

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This is purely a rename, so there is no functional change.

[rustbot]

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[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[lqd]

I guess coverage or coverage_graph could also be better than basic_coverage_blocks, but sure.

[Zalathar]

In the context of the InstrumentCoverage pass, everything is coverage, so just coverage would be confusing.

I can see an argument for coverage_graph, but in this context there's only one relevant graph, so I think the shorter name is worth the tradeoff. And coverage_graph also doesn't seem that helpful to readers who are unfamiliar with this code, because they would still have to look up the details of what the coverage graph actually is.