Fix incorrect cast in gtFoldExprConst

Fix incorrect cast in gtFoldExprConst

The fact that an operation is unsigned affects the operation itself but that doesn't mean that the result of the operation is also unsigned. Constants are stored as ssize_t and the node type is TYP_INT so the result has to be sign extended, not zero extended.

Otherwise code that uses the return of IconValue() without first narrowing it to int will behave incorrectly. Such code does exists, even gtFoldExprConst does this when folding shift operations:

uint a = uint.MaxValue;
uint b = 0;
int r = (int)checked(a + b);
Console.WriteLine((r >> 2).ToString("X"));

The above code prints 3FFFFFFF instead of the expected FFFFFFFF.

This also makes gtFoldExprConst consistent with ValueNumStore::EvalFuncForConstantArgs which evaluates TYP_INT nodes as int and then casts to ssize_t.

Commit migrated from https://github.com/dotnet/coreclr/commit/176e2c7964846bf5dea22557f0828475deac05e1