Rename getAddrModeImm12OpValue to getAddrModeImmOpValue and expand it to work

with immediates up to 16-bits in size. The same logic is applied to other LDR
encodings, e.g. VLDR, but which use a different immediate bit width (8-bits in
VLDR's case). Removing the "12" allows it to be more generic.

llvm-svn: 118094