[InstCombine] Handle vector compares in foldGEPIcmp()

Summary:
This is to fix PR38984 "InstCombine assertion at vector gep/icmp folding":
https://bugs.llvm.org/show_bug.cgi?id=38984

Reviewers: majnemer, spatel, lattner, lebedev.ri

Reviewed By: lebedev.ri

Subscribers: lebedev.ri, llvm-commits

Differential Revision: https://reviews.llvm.org/D52263

llvm-svn: 342647