Update the JIT to track `Span.Length` and `ReadOnlySpan.Length` as "never negative...

Update the JIT to track `Span.Length` and `ReadOnlySpan.Length` as "never negative" (#81055)

* Update the JIT to track `Span.Length` and `ReadOnlySpan.Length` as "never negative"

* Updating the "is never negative" info to be tracked in LclVarDsc

* Apply formatting patch

* Ensure lvIsNeverNegative is propagated to shadows

* Update src/coreclr/jit/gentree.h

Co-authored-by: SingleAccretion <62474226+SingleAccretion@users.noreply.github.com>
* Responding to PR feedback

* Adding two asserts

* Rename GenTreeField::IsNeverNegative to IsSpanLength and add a comment

* Fix an assert

Co-authored-by: SingleAccretion <62474226+SingleAccretion@users.noreply.github.com>