Rename getAddrModeImm12OpValue to getAddrModeImmOpValue and expand it to work

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@118094 91177308-0d34-0410-b5e6-96231b3b80d8