Fix mac_x64 in verified_mem mode; enforce fixed-length encoding (issue 21635)

Add optional argument to Assembler::Stop that forces a fixed-length instruction sequence.

TBR=iposva@google.com

Review URL: https://codereview.chromium.org//730173003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41790 260f80e4-7a28-3924-810f-c04153c831b5
Cleanup: remove flow-graph's cached copy of num stack locals as it can become stale if variables get added after flow graph is created.

R=johnmccutchan@google.com

Review URL: https://codereview.chromium.org//732203003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41791 260f80e4-7a28-3924-810f-c04153c831b5