math/big: avoid allocation in float.{Add, Sub} when there's no aliasing

math/big: avoid allocation in float.{Add, Sub} when there's no aliasing

name               old time/op    new time/op    delta
FloatAdd/10-4         116ns ± 1%      82ns ± 0%   -28.74%  (p=0.008 n=5+5)
FloatAdd/100-4        124ns ± 0%      86ns ± 1%   -30.34%  (p=0.016 n=4+5)
FloatAdd/1000-4       192ns ± 1%     123ns ± 0%   -35.94%  (p=0.008 n=5+5)
FloatAdd/10000-4      826ns ± 0%     438ns ± 0%   -46.99%  (p=0.000 n=4+5)
FloatAdd/100000-4    6.82µs ± 1%    3.36µs ± 0%   -50.74%  (p=0.008 n=5+5)
FloatSub/10-4         108ns ± 1%      77ns ± 1%   -29.06%  (p=0.008 n=5+5)
FloatSub/100-4        115ns ± 0%      79ns ± 0%   -31.48%  (p=0.029 n=4+4)
FloatSub/1000-4       168ns ± 0%      99ns ± 0%   -41.09%  (p=0.029 n=4+4)
FloatSub/10000-4      690ns ± 2%     288ns ± 1%   -58.24%  (p=0.008 n=5+5)
FloatSub/100000-4    5.37µs ± 1%    2.10µs ± 1%   -60.89%  (p=0.008 n=5+5)

name               old alloc/op   new alloc/op   delta
FloatAdd/10-4         48.0B ± 0%     0.0B ±NaN%  -100.00%  (p=0.008 n=5+5)
FloatAdd/100-4        64.0B ± 0%     0.0B ±NaN%  -100.00%  (p=0.008 n=5+5)
FloatAdd/1000-4        176B ± 0%       0B ±NaN%  -100.00%  (p=0.008 n=5+5)
FloatAdd/10000-4     1.41kB ± 0%   0.00kB ±NaN%  -100.00%  (p=0.008 n=5+5)
FloatAdd/100000-4    13.6kB ± 0%    0.0kB ±NaN%  -100.00%  (p=0.008 n=5+5)
FloatSub/10-4         48.0B ± 0%     0.0B ±NaN%  -100.00%  (p=0.008 n=5+5)
FloatSub/100-4        64.0B ± 0%     0.0B ±NaN%  -100.00%  (p=0.008 n=5+5)
FloatSub/1000-4        176B ± 0%       0B ±NaN%  -100.00%  (p=0.008 n=5+5)
FloatSub/10000-4     1.41kB ± 0%   0.00kB ±NaN%  -100.00%  (p=0.008 n=5+5)
FloatSub/100000-4    13.6kB ± 0%    0.0kB ±NaN%  -100.00%  (p=0.008 n=5+5)

Fixes #14868

Change-Id: Ia2b8b1a8ef0868288ecb25f812b17bd03ff40d1c
Reviewed-on: https://go-review.googlesource.com/23568
Reviewed-by: Robert Griesemer <gri@golang.org>