Add custom type for PseudoSourceValue

Add custom type for PseudoSourceValue

Summary:
PseudoSourceValue can be used to attach a target specific value for "well behaved" side-effects lowered from target specific intrinsics.
This is useful whenever there is not an LLVM IR Value around when representing such "well behaved" side-effected operations in backends by attaching a MachineMemOperand with a custom PseudoSourceValue as this makes the scheduler not treating them as "GlobalMemoryObjects" which triggers a logic that makes the operation act like a barrier in the Schedule DAG.

This patch adds another Kind to the PseudoSourceValue object which is "TargetCustom". It indicates a type of PseudoSourceValue that has a target specific meaning (aka. LLVM shouldn't assume any specific usage for such a PSV).

It supports the possibility of having many different kinds of "TargetCustom" PseudoSourceValues.

We had a discussion about if this was valuable or not (in particular because there was a believe that PSV were going away sooner or later) but seems like they are not going anywhere and I think they are useful backend side.

It is not clear the interaction of this with MIRParser (do we need a target hook to parse these?) and I would like a comment from Alex about that :)

Reviewers: arphaman, hfinkel, arsenm

Subscribers: Eugene.Zelenko, llvm-commits

Patch By: Marcello Maggioni

Differential Revision: https://reviews.llvm.org/D13575

llvm-svn: 290037