Fix wrong length passed to AAIOBE on x86/x64.

When getting the length of an array from memory (optimization
done by x86_memory_gen.cc), the slow path was incorrectly
unconditionnally treating the array as a string array, needing
to shr the length value.

Test: 667-out-of-bounds.
Change-Id: I274e68ecf63a70344c4f47db8431425433010283