Implement some basic simplifications involving min/max, for example

Implement some basic simplifications involving min/max, for example
max(a,b) >= a -> true.  According to my super-optimizer, these are
by far the most common simplifications (of the -instsimplify kind)
that occur in the testsuite and aren't caught by -std-compile-opts.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@130780 91177308-0d34-0410-b5e6-96231b3b80d8