[flang] Remove some needless operations in expr rewriting

Expressions emitted to module files and error messages
sometimes contain conversions of integer results of inquiry
intrinsics; these are usually not needed, and can conflict
with "int" in the user's namespace.  Improve folding so that
these conversions don't appear, and do some other clean-up
in adjacent code.

Differential Revision: https://reviews.llvm.org/D95172