[Sema] Handle large shift counts in GetExprRange (#68590)

GetExprRange did not expect that very large shift counts when
narrowing the range based on logical right shifts. So with inputs
such as
    *a >> 123456789012345678901uwb
it would hit assertions about trying to convert a too large APInt
into uint64_t.
This patch fixes that by using the APInt value when determining if
we should reduce the range by the shift count or not.