Rollup merge of #134551 - Zalathar:graph, r=lqd

coverage: Rename `basic_coverage_blocks` to just `graph`

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.

Author: dianqk