[AArch64] Fix encoding for lsl #12 in add/sub immediates

Whenever an add/sub immediate needs a fixup, we set that immediate field to zero,
which is correct, but we also set the shift bits to zero, which is not true for
instructions that use lsl #12. This patch makes sure that if lsl #12 was used,
it will appear in the encoding of the instruction.

Differential Revision: https://reviews.llvm.org/D23930

llvm-svn: 281898