NB in FE unsigned integer constants are now represented using signed
integer types (e.g., UInt constant actually holds an Int value).
So, in IR so far we also represent unsigned constant literals as
constant values of corresponding signed types, but with corresponding
unsigned type:
0xFFFF_FFFFu becomes 'CONST Int type=kotlin.UInt value=-1'
Dmitry Petrov
8999e5bfe6