Disable spilling into MMX registers
Implement more final operations in VectorTranslator

Most of this is pretty boring and mechanical. I added
VectorProp and VectorElem flags to help deal with the increasing
number of vector-related opcodes.
Filter strict_warnings like notices

There is a runtime option to filter out notices and warnings,
but strict_warnings were left out. Bundle them with notices.

We raise a lot of strict_warnings; and when we fix hphpiCompat
(to match zend better) we will raise a lot more, so this could
matter.
Fix ref generator parameters

Alias manager does not know whether generator parameters are passed by
reference. This didn't matter, because every generator had at least one
function call (hphp_continuation_done()) that pretty much disabled unused
variable elimination.

This diff fixes that, lets us get rid of artificial function calls in
generators and will allow later improvements in alias manager.
Introduce YieldExpression

Unhack the parser and introduce YieldExpression that emits the
equivalent set of opcodes that were emitted by bunch of
expressions/statements generated by parser before.

YieldExpression expects evaluation stack to contain just the value
being yielded, so {,List}AssignmentExpression need to evaluate RHS
first. The previous code had the same behavior.

This will let us consolidate continuation-related opcodes and make
them less tied with continuation objects.