[TTI] Fix getGEPCost() for geps with a single operand.

Previously this would sporadically crash as TargetType
was never initialized. We special-case the single-operand
case returning earlier and trying to mimic the behaviour of
isLegalAddressingMode as closely as possible.

Differential Revision:  https://reviews.llvm.org/D37277

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312357 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed