[x86] Hoist a function up to the rest of the non-type-specific lowering
helpers, and re-flow the logic to use early exit and be a bit more
readable.

No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218155 91177308-0d34-0410-b5e6-96231b3b80d8