[flang][hlfir] Simplify hlfir.assign default codegen for arrays

The previous code was always emitting two genAssign calls to create
a temporary copy of the RHS if it could overlap with the LHS.

This inline temporary creation is not needed anymore after:
https://github.com/llvm/llvm-project/commit/755535b5eb5f6d60e9cc347cecd9e057231b92bb
that updated the assignment runtime to detect overlap and make a
temporary copy in the runtime directly.

Note that optimized inlined assignment will still have to do the alias
analysis to skip the copy when added later.

Differential Revision: https://reviews.llvm.org/D144567