[clang][Interp] Fix compound assign operator types

Just like we do (or will do) for floating types, we need to take into
acocunt that the LHSComputationType, ResultType and type of the
expression (what we ultimately store) might be different.

Do this by emitting cast ops before and after doing the computation.

This fixes the test failures introduced by
490e8214 on big endian machines.

Differential Revision: https://reviews.llvm.org/D142328