Revert "ARM64: Use UntagSmi helpers and clean up assertions."

This reverts commit r22247 for breaking Linux arm64 build.

TBR=Jacob.Bramley@arm.com

Review URL: https://codereview.chromium.org/376553006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22249 ce2b1a6d-e550-0410-aec6-3dcde31c8c00