[X86][BtVer2] Fix latency of ALU RMW instructions.

Excluding ADC/SBB and the bit-test instructions (BTR/BTS/BTC), the observed
latency of all other RMW integer arithmetic/logic instructions is 6cy and not
5cy.

Example (ADD):

```
addb $0, (%rsp)            # Latency: 6cy
addb $7, (%rsp)            # Latency: 6cy
addb %sil, (%rsp)          # Latency: 6cy

addw $0, (%rsp)            # Latency: 6cy
addw $511, (%rsp)          # Latency: 6cy
addw %si, (%rsp)           # Latency: 6cy

addl $0, (%rsp)            # Latency: 6cy
addl $511, (%rsp)          # Latency: 6cy
addl %esi, (%rsp)          # Latency: 6cy

addq $0, (%rsp)            # Latency: 6cy
addq $511, (%rsp)          # Latency: 6cy
addq %rsi, (%rsp)          # Latency: 6cy
```

The same latency profile applies to SUB/AND/OR/XOR/INC/DEC.

The observed latency of ADC/SBB is 7-8cy. So we need a different write to model
those.  Latency of BTS/BTR/BTC is not fixed by this patch (they are much slower
than what the model for btver2 currently reports).

Differential Revision: https://reviews.llvm.org/D66636

llvm-svn: 369748