[dart2js] Fix slow analysis of word widths #50759 The `bitWidth` computation sometimes visits a large dependency DAG a tree. This change fixes the performance problem. - Cache results from previous visits. This fixes the occasional exponential time. - Move computation earlier 'instruction selection'. This allows the result to be used for detecting when assignment-ops can be used. I didn't see any changes due to this. - Strengthen bitWidth computation to fall back on type inference results. Bug: 50759 Change-Id: I482cc7d3b8a1e9af5798d22252cd9bc111be711d Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/276301 Commit-Queue: Stephen Adams <sra@google.com> Reviewed-by: Sigmund Cherem <sigmund@google.com>
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