For x86, canonicalize max

(x > y) ? x : y
=>
(x >= y) ? x : y

So for something like
(x - y) > 0 : (x - y) ? 0
It will be
(x - y) >= 0 : (x - y) ? 0

This makes is possible to test sign-bit and eliminate a comparison against
zero. e.g.
subl   %esi, %edi
testl  %edi, %edi
movl   $0, %eax
cmovgl %edi, %eax
=>
xorl   %eax, %eax
subl   %esi, $edi
cmovsl %eax, %edi

rdar://10633221

llvm-svn: 147512