[AArch64][SVE] Add patterns to select masked FP arith

Add patterns to select predicated instructions when lowering:

  fadd(a, select(mask, b, splat(0)))
  fsub(a, select(mask, b, splat(0)))

'fadd' is unsafe unless no-signed zeros fast-math flag is set, since

  -0.0 + 0.0 = 0.0

changes the sign. Alive2: https://alive2.llvm.org/ce/z/wbhJh_

Also adds FMA patterns for:

  fadd(a, select(mask, mul(b, c), splat(0))) -> fmla(a, mask, b, c)
  fsub(a, select(mask, mul(b, c), splat(0))) -> fmla(a, mask, b, c)

These patterns require the 'contract' fast-math flag to be set, and the
fadd 'nsz' as above.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D130564