[flang] Don't force SET_EXPONENT(I=...) argument to integer(4)

The implementation of the folding code for SET_EXPONENT() was written
in such a fashion as to convert the I= actual argument value to a 32-bit
integer.  Which is usually not a problem, but it's not always correct
and a test case ran into trouble with it.  Fix to allow any kind of
INTEGER without conversion.

Differential Revision: https://reviews.llvm.org/D135203