[X86] Remove RELEASE_ and ACQUIRE_ pseudo instructions. Use isel patterns and...

[X86] Remove RELEASE_ and ACQUIRE_ pseudo instructions. Use isel patterns and the normal instructions instead

At one point in time acquire implied mayLoad and mayStore as did release. Thus we needed separate pseudos that also carried that property. This appears to no longer be the case. I believe it was changed in 2012 with a comment saying that atomic memory accesses are marked volatile which preserves the ordering.

So from what I can tell we shouldn't need additional pseudos since they aren't carry any flags that are different from the normal instructions. The only thing I can think of is that we may consider them for load folding candidates in the peephole pass now where we didn't before. If that's important hopefully there's something in the memory operand we can check to prevent the folding without relying on pseudo instructions.

Differential Revision: https://reviews.llvm.org/D50212

llvm-svn: 338925