Fix incorrect redundant expression in target AMDGPU.

Summary:
The expression is detected as a redundant expression.
Turn out, this is probably a bug.

```
/home/etienneb/llvm/llvm/lib/Target/AMDGPU/SIInstrInfo.cpp:306:26: warning: both side of operator are equivalent [misc-redundant-expression]
  if (isSMRD(*FirstLdSt) && isSMRD(*FirstLdSt)) {
```

Reviewers: rnk, tstellarAMD

Subscribers: arsenm, cfe-commits

Differential Revision: http://reviews.llvm.org/D19460

llvm-svn: 267415