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[LV] Fold tail by masking - handle reductions

Authored by Ayal on Aug 28 2019, 2:02 AM.

Description

[LV] Fold tail by masking - handle reductions

Allow vectorizing loops that have reductions when tail is folded by masking.
A select is introduced in VPlan, choosing between the last value carried by the
loop-exit/live-out instruction of the reduction, and the penultimate value
carried by the reduction phi, according to the "i < n" mask of fold-tail.
This select replaces the last value as the live-out value of the loop.

Differential Revision: https://reviews.llvm.org/D66720

llvm-svn: 370173