[X86][BtVer2] Add a read-advance to every implicit register use of CMPXCHG8B/16B.

[X86][BtVer2] Add a read-advance to every implicit register use of CMPXCHG8B/16B.

This is a follow up of r369642.

This patch assigns a ReadAfterLd to every implicit register use of instruction
CMPXCHG8B and instruction CMPXCHG16B. Perf micro-benchmarks show that implicit
registers are read after 3cy from the start of execution.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369750 91177308-0d34-0410-b5e6-96231b3b80d8