Fix the performance of TryEmitConvert to String

[dadhi]

Before fix:

|                                               Method |      Mean |     Error |    StdDev | Ratio | RatioSD |  Gen 0 |  Gen 1 |  Gen 2 | Allocated |
|----------------------------------------------------- |----------:|----------:|----------:|------:|--------:|-------:|-------:|-------:|----------:|
|                CreateLightExpression_and_CompileFast | 12.954 us | 0.2578 us | 0.3859 us |  1.00 |    0.00 | 1.8005 | 0.8850 | 0.0610 |    5.6 KB |
| CreateLightExpression_and_CompileFast_with_intrinsic |  8.033 us | 0.1321 us | 0.1764 us |  0.62 |    0.02 | 0.8698 | 0.4272 | 0.0458 |   2.67 KB |        

After fix:

|                                               Method |     Mean |     Error |    StdDev | Ratio | RatioSD |  Gen 0 |  Gen 1 |  Gen 2 | Allocated |
|----------------------------------------------------- |---------:|----------:|----------:|------:|--------:|-------:|-------:|-------:|----------:|
|                CreateLightExpression_and_CompileFast | 7.172 us | 0.1428 us | 0.1955 us |  1.00 |    0.00 | 0.4578 | 0.2289 | 0.0305 |   2.84 KB |
| CreateLightExpression_and_CompileFast_with_intrinsic | 6.492 us | 0.1255 us | 0.1289 us |  0.91 |    0.03 | 0.4349 | 0.2136 | 0.0305 |   2.67 KB |