[InstCombine] move icmp transforms that might be recognized as min/max and inf-loop (PR31751)

This is a minimal patch to avoid the infinite loop in:
https://llvm.org/bugs/show_bug.cgi?id=31751

But the general problem is bigger: we're not canonicalizing all of the min/max forms reported
by value tracking's matchSelectPattern(), and we don't define min/max consistently. Some code
uses matchSelectPattern(), other code uses matchers like m_Umax, and others have their own
inline definitions which may be subtly different from any of the above.

The reason that the test cases in this patch need a cast op to trigger is because we don't
(yet) canonicalize all min/max forms based on matchSelectPattern() in 
canonicalizeMinMaxWithConstant(), but we do make min/max+cast transforms based on 
matchSelectPattern() in visitSelectInst().

The location of the icmp transforms that trigger the inf-loop seems arbitrary at best, so
I'm moving those behind the min/max fence in visitICmpInst() as the quick fix.

llvm-svn: 293345