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[InstCombine] Add fold for extracting known elements from a stepvector

Authored by CarolineConcatto on May 26 2021, 1:50 AM.

Description

[InstCombine] Add fold for extracting known elements from a stepvector

This patch allows folding stepvector + extract to the lane when the lane is
lower than the minimum size of the scalable vector. This fold is possible
because lane X of a stepvector is also X!
For instance, extracting element 3 of a <vscale x 4 x i64>stepvector is 3.

Differential Revision: https://reviews.llvm.org/D103153