[ARM] Add fp16 addressing to isLegalT2AddressImmediate

The fp16 version of VLDR takes a imm8 multiplied by 2. This updates the costs
to account for those, and adds extra testing. It is dependant upon hasFPRegs16
as this is what the load/store instructions require.

Differential Revision: https://reviews.llvm.org/D62966

llvm-svn: 362872