RFR: 8356891: Some code simplifications for basic BigIntegers' bit operations

[fabioromano1]

On Sun, 11 May 2025 16:12:38 GMT, fabioromano1 <duke at openjdk.org> wrote:

>>> This should really be refactored to an instance method `bitLengthUnsigned` or `magBitCount`
>> 
>> I would prefer `magBitLength` as a name, since `bitCount` is usually referred to one's bit count. But before do this, I'd hear the opinion of @rgiulietti about that.
>
>> Making this an instance method makes code cleaner and allows us to cache if we find this calculation expensive.
> 
> I recall that `bitLength` is already cached, so it would be preferable to replace `bitLength`'s cache with `magBitLength`'s cache if we want to do so.

> the `len` is always `val.length` and the `val` is always the `mag` array of some `BigInteger` from somewhere.

Yes, but making it an instance method could be too narrowing, as sometimes in the code `int[]` array local variables are used in preference of `mag` instance fields.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25166#discussion_r2083567353