Thumb2 add immediate alias for SP

The Thumb2 add immediate is in fact defined for SP. The manual is misleading as it points to a different section for add immediate with SP, however the encoding is the same as for add immediate with register only with the SP operand hard coded. As such add immediate with SP and add immediate with register can safely be treated as the same instruction.

All the patch does is adjust a register constraint on an instruction alias.

llvm-svn: 188676