It turns out that code deleted in 9a05aea8 actually had a legitimate purpose, so it is added back, this time with more extensive comments justifying it.

Also, takes the instruction's IsDestNonKillable flag into account when updating the live register usage count (along with extra comments on why that is necessary).

Furthermore, removes an unnecessary assert that otherwise fails when --asm-verbose is used with --filetype=iasm or --filetype-obj.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/1158113002