ARM64/x86-64: Fix mterp fill-array-data-payload pointer calculation.

Fix the pointer calculation to sign-extend the offset
instead of zero-extending it, just like we do for the switch
table pointer calculation. Clean up comments for the switch.

Test: Additional test in 412-new-array.
Change-Id: Ibb1d2d3fcb109f59280aca08de21e42edc4ce66b
9 files changed