Run underscore validation only if we have seen underscores.

Some performance results on my laptop:
name                   old time/op  new time/op  delta
Atof64Decimal-12       30.5ns ± 0%  23.8ns ± 0%  -22.02%  (p=0.016 n=5+4)
Atof64Float-12         39.0ns ± 0%  28.7ns ± 0%  -26.39%  (p=0.002 n=6+6)
Atof64FloatExp-12      64.4ns ± 1%  54.4ns ± 1%  -15.65%  (p=0.002 n=6+6)
Atof64Big-12            115ns ± 1%    87ns ± 1%  -24.45%  (p=0.002 n=6+6)
Atof64RandomBits-12     187ns ±14%   156ns ±19%  -16.46%  (p=0.032 n=6+6)
Atof64RandomFloats-12   126ns ± 0%   105ns ± 1%  -16.65%  (p=0.000 n=6+5)
Atof32Decimal-12       32.0ns ± 1%  24.0ns ± 1%  -24.97%  (p=0.002 n=6+6)
Atof32Float-12         37.1ns ± 1%  27.0ns ± 1%  -27.42%  (p=0.002 n=6+6)
Atof32FloatExp-12      68.4ns ± 1%  54.2ns ± 1%  -20.77%  (p=0.002 n=6+6)
Atof32Random-12        92.0ns ± 1%  77.4ns ± 0%  -15.81%  (p=0.000 n=6+5)
ParseInt/Pos/7bit-12   19.4ns ± 1%  13.8ns ±10%  -28.94%  (p=0.002 n=6+6)
ParseInt/Pos/26bit-12  29.1ns ± 1%  19.8ns ± 2%  -31.92%  (p=0.002 n=6+6)
ParseInt/Pos/31bit-12  33.1ns ± 0%  22.3ns ± 3%  -32.62%  (p=0.004 n=5+6)
ParseInt/Pos/56bit-12  47.8ns ± 1%  30.7ns ± 1%  -35.78%  (p=0.004 n=6+5)
ParseInt/Pos/63bit-12  51.9ns ± 1%  33.4ns ± 2%  -35.49%  (p=0.002 n=6+6)
ParseInt/Neg/7bit-12   18.5ns ± 4%  13.4ns ± 3%  -27.88%  (p=0.002 n=6+6)
ParseInt/Neg/26bit-12  28.4ns ± 3%  19.7ns ± 3%  -30.38%  (p=0.002 n=6+6)
ParseInt/Neg/31bit-12  31.9ns ± 1%  21.8ns ± 2%  -31.56%  (p=0.002 n=6+6)
ParseInt/Neg/56bit-12  46.2ns ± 0%  30.6ns ± 1%  -33.73%  (p=0.004 n=5+6)
ParseInt/Neg/63bit-12  50.2ns ± 1%  33.2ns ± 1%  -33.96%  (p=0.002 n=6+6)

Fixes #33330

Change-Id: I119da66457c2fbaf6e88bb90cf56417a16df8f0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/187957
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>