[AArch64][SVE] Asm: Support for reversed subtract (SUBR) instruction.

                                                                           
This patch adds both a vector and an immediate form, e.g.                  
                                                                           
- Vector form:                                                             
                                                                           
    subr z0.h, p0/m, z0.h, z1.h                                            
                                                                           
  subtract active elements of z0 from z1, and store the result in z0.      
                                                                           
- Immediate form:                                                          
                                                                           
    subr z0.h, z0.h, #255                                                  
                                                                           
  subtract elements of z0, and store the result in z0.

llvm-svn: 336274