Narrow right shifts need to encode their immediates differently from a normal

shift.

   16-bit: imm6<5:3> = '001', 8 - <imm> is encded in imm6<2:0>
   32-bit: imm6<5:4> = '01',16 - <imm> is encded in imm6<3:0>
   64-bit: imm6<5> = '1', 32 - <imm> is encded in imm6<4:0>

llvm-svn: 126723