[AArch64][SVE] Asm: Support for FP conversion instructions.

The variants added are:

- fcvt   (FP convert precision)
- scvtf  (signed int -> FP) 
- ucvtf  (unsigned int -> FP) 
- fcvtzs (FP -> signed int (round to zero))
- fcvtzu (FP -> unsigned int (round to zero))

For example:
  fcvt   z0.h, p0/m, z0.s  (single- to half-precision FP) 
  scvtf  z0.h, p0/m, z0.s  (32-bit int to half-precision FP) 
  ucvtf  z0.h, p0/m, z0.s  (32-bit unsigned int to half-precision FP) 
  fcvtzs z0.s, p0/m, z0.h  (half-precision FP to 32-bit int)
  fcvtzu z0.s, p0/m, z0.h  (half-precision FP to 32-bit unsigned int)

llvm-svn: 336265