Return an error when a bitSize below 0 or above 64 is specified.

Move bitSize 0 handling in ParseInt after the call to ParseUint
to avoid a spill.

AMD64:
name       old time/op  new time/op  delta
Atoi       28.9ns ± 6%  27.4ns ± 6%  -5.21%  (p=0.002 n=20+20)
AtoiNeg    24.6ns ± 2%  23.1ns ± 1%  -6.04%  (p=0.000 n=19+18)
Atoi64     38.8ns ± 1%  38.0ns ± 1%  -2.03%  (p=0.000 n=17+20)
Atoi64Neg  35.5ns ± 1%  34.3ns ± 1%  -3.42%  (p=0.000 n=19+20)

Updates #21275

Change-Id: I70f0e4a16fa003f7ea929ca4ef56bd1a4181660b
Reviewed-on: https://go-review.googlesource.com/55139Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>