Optimize the code-gen for Math.pow(x, 0.5)

Math.pow(x, 0.5) --> Math.sqrt(x)

Before:
```
Benchmark (seed) Mode Cnt Score Error Units
MathBench.powDouble0Dot5 0 thrpt 8 45525.117 ? 11.686 ops/ms
MathBench.powDouble0Dot5Loop 0 thrpt 8 0.031 ? 0.001 ops/ms

Benchmark (seed) Mode Cnt Score Error Units
MathBench.powDouble0Dot5 0 thrpt 8 45509.317 ? 6.581 ops/ms
MathBench.powDouble0Dot5Loop 0 thrpt 8 0.031 ? 0.001 ops/ms
```

After:
```
Benchmark (seed) Mode Cnt Score Error Units
MathBench.powDouble0Dot5 0 thrpt 8 343354.892 ? 362.900 ops/ms
MathBench.powDouble0Dot5Loop 0 thrpt 8 0.457 ? 0.001 ops/mso

Benchmark (seed) Mode Cnt Score Error Units
MathBench.powDouble0Dot5 0 thrpt 8 343421.559 ? 49.326 ops/ms
MathBench.powDouble0Dot5Loop 0 thrpt 8 0.457 ? 0.001 ops/ms
```