[EarlyCSE] fold commutable intrinsics

Handling the new min/max intrinsics is the motivation, but it
turns out that we have a bunch of other intrinsics with this
missing bit of analysis too.

The FP min/max tests show that we are intersecting FMF,
so that part should be safe too.

As noted in https://llvm.org/PR46897 , there is a commutative
property specifier for intrinsics, but no corresponding function
attribute, and so apparently no uses of that bit. We may want to
remove that next.

Follow-up patches should wire up the Instruction::isCommutative()
to this IntrinsicInst specialization. That requires updating
callers to be aware of the more general commutative property
(not just binops).

Differential Revision: https://reviews.llvm.org/D86798