This CL fixes a bug found by Clusterfuzz, in which the functions
LoadDataViewByteOffset and -ByteLength incorrectly had a return
type of TNode<Smi> instead of TNode<Number>.

This caused a CAST() call to fail when the requested byte offset
or byte length did not fit inside a Smi, i.e. when the underlying
ArrayBuffer of the DataView had a length longer than 2^30 on
32-bit platforms.

The CL also includes a new test in mjsunit to test against this.

Bug: chromium:869313
Change-Id: Ibb7d29bda5782a12c4b506c070bb03fef8c3ec70
Reviewed-on: https://chromium-review.googlesource.com/1158582
Commit-Queue: Théotime Grohens <theotime@google.com>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54900}