bsls_performancehint.t: Fix unused and undefined behavior warnings.

In addition to fixing unused variable warnings, fixes:

```
../groups/bsl/bsls/bsls_performancehint.t.cpp:295: warning: cast from type ‘volatile int*’ to type ‘int*’ casts away constness
../groups/bsl/bsls/bsls_performancehint.t.cpp:297: warning: cast from type ‘volatile int*’ to type ‘int*’ casts away constness
```

by adding a `const_cast` and fixes:

```
../groups/bsl/bsls/bsls_performancehint.t.cpp:277: warning: operation on ‘arrayA’ may be undefined
../groups/bsl/bsls/bsls_performancehint.t.cpp:278: warning: operation on ‘arrayA’ may be undefined
../groups/bsl/bsls/bsls_performancehint.t.cpp:279: warning: operation on ‘arrayA’ may be undefined
../groups/bsl/bsls/bsls_performancehint.t.cpp:280: warning: operation on ‘arrayA’ may be undefined
../groups/bsl/bsls/bsls_performancehint.t.cpp:282: warning: operation on ‘arrayA’ may be undefined
../groups/bsl/bsls/bsls_performancehint.t.cpp:283: warning: operation on ‘arrayA’ may be undefined
../groups/bsl/bsls/bsls_performancehint.t.cpp:284: warning: operation on ‘arrayA’ may be undefined
../groups/bsl/bsls/bsls_performancehint.t.cpp:285: warning: operation on ‘arrayA’ may be undefined
```

by using `+=` to avoid undefined behavior by incrementing and referencing `arrayA` in the same statement.