[flang] Avoid unnecessary temporaries in ArrayValueCopy.

Assume no conflict between pointer arrays and arrays without the target
attribute, if the fact of an array not having the target attribute
can be reliably computed.

This change speeds up SPEC CPU2017/527.cam from 2.5k seconds to 880 seconds
on Icelake, and makes further performance investigation easier.

Differential Revision: https://reviews.llvm.org/D142273