Bound Check for arr[(X & 0x3) << 4] pattern

Another minimal repro from base64 encoder:
```cs
byte Foo(byte inData)
{
    ReadOnlySpan<byte> base64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/="u8;
    return base64[(inData & 0x03) << 4];
}
```
`(X & 3)` means `X` is `[0..3]` and `<< 4` means the max value is `48` while the array has 65 elements => the bound check should be elided.