[AArch64][SVE] Asm: Support for predicated FP operations (FP immediate)

This patch completes support for the following floating point
instructions that take FP immediates:
  FADD*  (addition)
  FSUB   (subtract)
  FSUBR  (subtract reverse form)
  FMUL*  (multiplication)
  FMAX*  (maximum)
  FMAXNM (maximum number)
  FMIN   (maximum)
  FMINNM (maximum number)

All operations are predicated and take a FP immediate operand,
e.g.

  fadd z0.h, p0/m, z0.h, #0.5
  fmin z0.s, p0/m, z0.s, #1.0
        ^___________^ (tied)

* Instructions added in a previous patch.

llvm-svn: 337272