Support MemoryLocation::UnknownSize in TargetLowering::IntrinsicInfo

Summary:
Previously IntrinsicInfo::size was an unsigned what can't represent the
64 bit value used by MemoryLocation::UnknownSize.

Reviewers: jmolloy

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68219

llvm-svn: 373214