[SCEV] Canonicalize ext(min/max(x, y)) to min/max(ext(x), ext(y))

I stumbled over this while trying to improve our exit count work. These expressions
are equivalent for complementary signed/unsigned ext and min/max (including umin_seq),
but they are not canonicalized and SCEV cannot recognize them as the same.

The benefit of this canonicalization is that SCEV can prove some new equivalences which
it coudln't prove because of different forms. There is 1 test where trip count seems pessimized,
I could not directly figure out why, but it just seems an unrelated issue that we can fix.
Other changes seem neutral or positive to me.

Differential Revision: https://reviews.llvm.org/D141481
Reviewed By: nikic