[ValueTracking] recognize min/max of min/max patterns

This is part of solving PR35717:
https://bugs.llvm.org/show_bug.cgi?id=35717

The larger IR optimization is proposed in D41603, but we can show 
the improvement in ValueTracking using codegen tests because 
SelectionDAG creates min/max nodes based on ValueTracking. 

Any target with min/max ops should show wins here. I chose AArch64
vector ops because they're clean and uniform.

Some Alive proofs for the tests (can't put more than 2 tests in 1 
page currently because the web app says it's too long):
https://rise4fun.com/Alive/WRN
https://rise4fun.com/Alive/iPm
https://rise4fun.com/Alive/HmY
https://rise4fun.com/Alive/CNm
https://rise4fun.com/Alive/LYf

llvm-svn: 321672