[VPlan] Introduce VPWidenSelectRecipe (NFC).

Authored by fhahn on Apr 13 2020, 12:28 AM.


[VPlan] Introduce VPWidenSelectRecipe (NFC).

Widening a selects depends on whether the condition is loop invariant or
not. Rather than checking during codegen-time, the information can be
recorded at the VPlan construction time.

This was suggested as part of D76992, to reduce the reliance on
accessing the original underlying IR values.

Reviewers: gilr, rengolin, Ayal, hsaito

Reviewed By: gilr

Differential Revision: https://reviews.llvm.org/D77869