[SLP] Fix assert from non-constant index in insertelement

A call to getInsertIndex() in getTreeCost() is returning None,
which causes an assert because a non-constant index value for
insertelement was not expected. This case occurs when the
insertelement index value is defined with a PHI.

Differential Revision: https://reviews.llvm.org/D120223

(cherry picked from commit 3cc15e2c)