Fixed bad assert

UInt <-> Int conversions are noop (bitwise), so it's allowed
to simply perform a move operation in that case.

Change-Id: I2078b2cf2933f5ced25b8ea81933e543a017faaf
Reviewed-on: https://swiftshader-review.googlesource.com/5143
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>