[X86] Refactor X86ISelDAGToDAG::SelectAtomicLoadArith - NFC

Summary:
Mostly renaming the (not very explicit) variables Tmp0, .. Tmp4, and grouping
related statements together, along with a few lines of comments for the
surprising parts.

No functional change intended.

Test Plan: make check-all

Reviewers: jfb

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D5088