Rollup merge of #127252 - fitzgen:edge-cases-for-bitwise-operations, r=m-ou-se

Add edge-case examples to `{count,leading,trailing}_{ones,zeros}` methods

Some architectures (i386) do not define a "count leading zeros" instruction, they define a "find first set bit" instruction (`bsf`) whose result is undefined when given zero (ie none of the bits are set). Of this family of bitwise operations, I always forget which of these things is potentially undefined for zero, and I'm also not 100% sure that Rust provides a hard guarantee for the results of these methods when given zero. So I figured there are others who have these same uncertainties, and it would be good to resolve them and answer the question via extending these doc examples/tests.

See https://en.wikipedia.org/wiki/Find_first_set#Hardware_support for more info on i386 and `bsf` on zero.

Author: matthiaskrgr