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

@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.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D133666