[PATCH] [X86] Allow atomic operations using immediates to avoid using a register

[Robin Morisset]

Actually, on X86 the xchg instruction is a bit special in that it automatically behaves as if it had a LOCK prefix, even if it is not explicit (source: Intel manual, 2B 4-71)
So I don't think it is a bug that it is not added.
Do you think I should add a comment somewhere to that effect? And if so where?

Thanks again for your reviews!

http://reviews.llvm.org/D4796