[InstCombine] Handle vector gep with scalar argument in evaluateInDifferentElementOrder

[InstCombine] Handle vector gep with scalar argument in evaluateInDifferentElementOrder

Summary:
This fixes PR41270.

The recursive function evaluateInDifferentElementOrder expects to be called
on a vector Value, so when we call it on a vector GEP's arguments, we must
first check that the argument is indeed a vector.

Reviewers: reames, spatel

Reviewed By: spatel

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60058

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@357389 91177308-0d34-0410-b5e6-96231b3b80d8