[VPlan] Add VPValue::isDefinedOutsideVectorRegions helper (NFC).

[fhahn]

Summary

@Ayal suggested a better named helper than using !getDef() to check if
a value is invariant across all parts.

The property we are using here is that the VPValue is defined outside
any vector loop region. There's a TODO left to handle recipes defined in
pre-header blocks.

[reames]

llvm/lib/Transforms/Vectorize/VPlan.h

3058

With this placement, this is equivalent to return false. The interesting case is the return true below.

Maybe:

if (VPV->isDefinedOutsideVectorRegion())
  return true;
auto *Def = VPV->getDef();
assert(Def);  
...

[Ayal]

llvm/lib/Transforms/Vectorize/VPlanValue.h

205

Thanks for following up!
Perhaps something like "isaVPLiveIn()" analogous to isa<VPLiveOut>()? Both are saying it's a wrapper of an IR value/user outside the scope of VPlan rather than a recipe inside this scope.
More precise type information could be derived from the wrapped underlying IR, or implied assuming that VPlan is only fed-by and feeds scalars from/to IR.

[fhahn]

Reorder code as suggested, apologies for the delay

[fhahn]

llvm/lib/Transforms/Vectorize/VPlan.h

3058

Thanks, re-ordered the code!

llvm/lib/Transforms/Vectorize/VPlanValue.h

205

The helper is intended to go beyond pure live-ins in the future. In particular, the current definition could also return true for recipes defined in the pre-header, as in the code below. I wasn't able to come up with a test case to trigger this code though for now. Does that make sense?

+bool VPValue::isDefinedOutsideVectorRegions() const {
+  const VPRecipeBase *Def = getDef();
+  if (!Def)
+    return true;
+  const VPBasicBlock *DefBlock = Def->getParent();
+  const VPlan &Plan = *DefBlock->getPlan();
+  return Plan.getEntry() == DefBlock;
+}
+

[reames]

LGTM