fast-mode compiler.

1. We avoid generating a useless temporary for assignments with
nontrivial right-hand sides.  Instead of translating id = expr into:

...
tmp = <last expr instruction>
id = tmp

we generate directly

...
id = <last expr instruction>

by passing a data destination ('hint') down the AST.  The semantics is
to use the destination as a result location if a temp is needed.  It
may be ignored.  NULL indicates I don't care and you should generate a
temp.

2. We correctly handle assignments as subexpressions.  When building
the CFG for an expression we accumulate the assigned variables and we
emit a move to a fresh temporary if a value in a variable is in
jeopardy of being overwritten.

Review URL: http://codereview.chromium.org/165056

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2643 ce2b1a6d-e550-0410-aec6-3dcde31c8c00