Currently, Generate_ArrayIndexOf handles the hypothetical case of an array with
a fast ElementsKind and non-SMI length. This should not happen (and is checked
against in JSArray::JSArrayVerify of objects_debug.cc).

Therefore this CL replaces that handling with a CSA_ASSERT that the length is
indeed SMI.

The CL also simplifies loading of the (SMI) length on 64 bit architectures by
using LoadAndUntagObjectField instead of LoadObjectField+SmiToWord.

The CL does not add new tests, because test/mjsunit/array-length.js should
cover this already.

BUG=v8:5985

Review-Url: https://codereview.chromium.org/2714173002
Cr-Commit-Position: refs/heads/master@{#43431}