[CodeGen] Fix incorrect insertion point selection for reduction nodes in ComplexDeinterleavingPass

When replacing ComplexDeinterleavingPass::ReductionOperation, we can do it
either from the Real or Imaginary part. The correct way is to take whichever
is later in the BasicBlock, but before the patch, we just always took the
Real part.

Fixes https://github.com/llvm/llvm-project/issues/65044

Differential Revision: https://reviews.llvm.org/D159209