[SLP] Fix crash after r358519, by V. Porpodas.

Summary: The code did not check if operand was undef before casting it to Instruction.

Reviewers: RKSimon, ABataev, dtemirbulatov

Reviewed By: ABataev

Subscribers: uabelho

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61024

llvm-svn: 359136