VRP for array casts

[dlangBugzillaToGithub]

anonymous4 reported this on 2018-10-18T09:47:47Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=19310

Description

long[] f(byte[] a)
{
    return cast(long[])a[0..a.length/8*8];
}

Here the compiler can see that the array length is multiple of 8 and simply divide the length by 8 for the cast.