enforce canonicalization of value_range's

[Aldy Hernandez]

This is a ping of:

https://gcc.gnu.org/ml/gcc-patches/2019-07/msg00008.html

I have addressed the comments Jeff mentioned there.

This patch goes before the VR_VARYING + types patch, but I can adapt 
either one to come first.  I can even munge them together into one 
patch, if it helps review.

Tested on x86-64 Linux.

OK (assuming ChangeLog entries)?

Aldy

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