add assert_true()

This is an assert that is active in debug mode.  For the moment it only
works in the interpreter, but I plan to follow up with JIT code too.

assert_true() is a data sink like a store() as far as lifetime goes,
though we take care to allow it to be hoisted if its inputs are.  An
assert_true's existence will keep all its inputs alive, and in release
builds where we skip the instruction, those inputs will all drop away
automatically.

Tested locally by forcing the interpreter.  It shouldn't be long before
I have at least x86 JIT asserts working too.

Change-Id: I7aba40d040436a57a6b930790f7b8962bafb1a8c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/253756
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
4 files changed