The -asm-verbose flag adds comments to the text asm output about register availability.  Specifically, it prints the registers in use at the beginning and end of each block, and it prints which registers' live ranges end at each instruction.

This is extremely helpful when studying the output to find opportunities to improve the code quality.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/682983004