Avoid an allocation in BigInteger.Parse("-2147483648")

[kzrnm]

Possibly related to #97589 (comment)

Before the change, this = s_bnMinInt; was unreachable.

Benchmark

[DisassemblyDiagnoser]
[GroupBenchmarksBy(BenchmarkLogicalGroupRule.ByMethod)]
public class ParseSmall
{
    [Params(
        "-2147483648",
        "-2147483647",
        "2147483647",
        "2147483648"
    )]
    public string N;

    [Benchmark] public BigInteger Parse() => BigInteger.Parse(N);
}

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3810/23H2/2023Update/SunValley3)
13th Gen Intel Core i5-13500, 1 CPU, 20 logical and 14 physical cores
.NET SDK 9.0.100-preview.5.24307.3
  [Host]  : .NET 9.0.0 (9.0.24.30607), X64 RyuJIT AVX2
  LongRun : .NET 9.0.0 (42.42.42.42424), X64 RyuJIT AVX2

Job=LongRun  IterationCount=100  LaunchCount=3  
WarmupCount=15  

Method	Toolchain	N	Mean	Error	StdDev	Median	Ratio	RatioSD	Code Size	Gen0	Allocated	Alloc Ratio
Parse	\main\corerun.exe	-2147483647	54.29 ns	0.118 ns	0.599 ns	54.10 ns	1.00	0.00	1,226 B	-	-	NA
Parse	\pr\corerun.exe	-2147483647	53.41 ns	0.053 ns	0.270 ns	53.39 ns	0.98	0.01	1,226 B	-	-	NA
Parse	\main\corerun.exe	-2147483648	58.50 ns	0.144 ns	0.740 ns	58.67 ns	1.00	0.00	1,226 B	0.0025	32 B	1.00
Parse	\pr\corerun.exe	-2147483648	55.90 ns	0.067 ns	0.346 ns	55.84 ns	0.96	0.02	1,226 B	-	-	0.00
Parse	\main\corerun.exe	2147483647	52.68 ns	0.048 ns	0.242 ns	52.66 ns	1.00	0.00	1,226 B	-	-	NA
Parse	\pr\corerun.exe	2147483647	52.39 ns	0.070 ns	0.358 ns	52.32 ns	0.99	0.01	1,226 B	-	-	NA
Parse	\main\corerun.exe	2147483648	58.38 ns	0.187 ns	0.954 ns	58.12 ns	1.00	0.00	1,226 B	0.0025	32 B	1.00
Parse	\pr\corerun.exe	2147483648	57.65 ns	0.097 ns	0.491 ns	57.59 ns	0.99	0.02	1,226 B	0.0025	32 B	1.00

[danmoseley]

@tannergooding still OK to merge?