ARM PKH shift ammount operand printing tweaks.

Move the shift operator and special value (32 encoded as 0 for PKHTB) handling
into the instruction printer. This cleans up a bit of the disassembler
special casing for these instructions, more easily handles not printing the
operand at all for "lsl #0" and prepares for correct asm parsing of these
operands.

llvm-svn: 135626