ARM cost model: Correct cost for some cheap float to integer conversions

Fix cost of some "cheap" cast instructions. Before this patch we used to
estimate for example:
  cost of 16 for instruction:   %r = fptoui <4 x float> %v0 to <4 x i16>

While we would emit:
  vcvt.s32.f32  q8, q8
  vmovn.i32 d16, q8
  vuzp.8  d16, d17

All other costs are left to the values assigned by the fallback logic. Theses
costs are mostly reasonable in the sense that they get progressively more
expensive as the instruction sequences emitted get longer.

radar://13434072

llvm-svn: 177333