[AArch64][SVE] Asm: Support for unpredicated FP operations.

This patch adds support for the following unpredicated
floating-point instructions:

  FADD      Floating point add
  FSUB      Floating point subtract
  FMUL      Floating point multiplication
  FTSMUL    Floating point trigonometric starting value
  FRECPS    Floating point reciprocal step
  FRSQRTS   Floating point reciprocal square root step

The instructions have the following assembly format:
  fadd z0.h, z1.h, z2.h
and have variants for 16, 32 and 64-bit FP elements.

llvm-svn: 337383