The MC code couldn't handle ARM LDR instructions with negative offsets:

    vldr.64 d1, [r0, #-32]

The problem was with how the addressing mode 5 encodes the offsets. This change
makes sure that the way offsets are handled in addressing mode 5 is consistent
throughout the MC code. It involves re-refactoring the "getAddrModeImmOpValue"
method into an "Imm12" and "addressing mode 5" version. But not to worry! The
majority of the duplicated code has been unified.

llvm-svn: 118144