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[SCEV] Fix false-positive recognition of simple recurrences. PR49856

Authored by mkazantsev on Apr 6 2021, 11:14 PM.

Description

[SCEV] Fix false-positive recognition of simple recurrences. PR49856

A value from reachable block may come to a Phi node as its input from
unreachable block. This may confuse matchSimpleRecurrence which
has no access to DomTree and can falsely recognize something as a recurrency
because of this effect, as the attached test shows.

Patch ae7b1e deals with half of this problem, but it only accounts from
the case when an unreachable instruction comes to Phi as an input.

This patch provides a generalization by checking that no Phi block's
predecessor is unreachable (no matter what the input is).

Differential Revision: https://reviews.llvm.org/D99929
Reviewed By: reames