Optimize counting digits in line numbers during error reporting further
This one-ups #82248 by switching the strategy: Instead of dividing the value by 10 repeatedly, we compare with a limit that we multiply by 10 repeatedly. In my benchmarks, this took between 50% and 25% of the original time. The reasons for being faster are:
1. While LLVM is able to replace a division by constant with a multiply + shift, a plain multiplication is still faster. However, this doesn't even factor, because
2. Multiplication, unlike division, is const. We also use a simple for-loop instead of a more complex loop + break, which allows
3. rustc to const-fold the whole loop, and indeed the assembly output simply shows a series of comparisons.
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