[ARM] Add MemOperand to LDRcp to enable DCE.

LDRcp should be deleted when the dest register is dead in register
coalescing. Without MemOp, dead LDRcp will cause dead constant pool
value which references to non-existing label.

Patch by Yin Ma.

Differential Revision: https://reviews.llvm.org/D54173

llvm-svn: 346563