[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