[DAGCombiner] don't dereference an operand that doesn't exist (PR26070)

The bug was introduced with changes for x86-64 fp128:
http://reviews.llvm.org/rL254653

I don't know why an x86 change is here, so I'll follow up in:
http://reviews.llvm.org/D15134

Should fix:
https://llvm.org/bugs/show_bug.cgi?id=26070

llvm-svn: 257200