[opt] Disincentivize new tests from using old pass syntax

Over the past day or so, i've took a large swing at our tests,
and reduced the number of tests that were still using the old syntax
from ~1800 to just 200.

Left to handle: (as it is seen in this patch)
* Transforms/LSR
* Transforms/CGP
* Transforms/TypePromotion
* Transforms/HardwareLoops
* Analysis/*
* some misc.

I think this is the right point to start actively refusing
to honor the old syntax, except for the old tests,
to prevent the old syntax from creeping back in.

Thus, let's add temporary default-off flag,
and if it is not passed refuse to accept old syntax.
The tests that still need porting are annotated with this flag.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D139647