[release/7.0-rc2] Fixing SpanHelpers.LastIndexOfAnyValueType to no longer create out of bounds GC refs (#75885)

* Fixing SpanHelpers.LastIndexOfAnyValueType to no longer create out of bounds GC refs

* Apply suggestions from code review

Co-authored-by: Adam Sitnik <[email protected]>

* Adjusting the comment as per PR review feedback

Co-authored-by: Tanner Gooding <[email protected]>
Co-authored-by: Adam Sitnik <[email protected]>

Author: 3 people