[dart2wasm] Use unsigned comparison in native typed array index checks

wasm-opt is not able to do this optimization and using one unsigned
comparison instead of two comparisons saves some binary space and runs
faster.

(JS typed array classes already do this)

Change-Id: Ia51d0d501af9bfd119b53d2373456bd4f0c36f25
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/335825
Commit-Queue: Ömer Ağacan <[email protected]>
Reviewed-by: Aske Simon Christensen <[email protected]>