[AMDGPU][LoopUnroll] Increase BB size to analyze for complete unroll.

The `UnrollMaxBlockToAnalyze` parameter is used at the stage when we have no
information about a loop body BB cost. In some cases, e.g. for simple loop
```
for(int i=0; i<32; ++i){
  D = Arr2[i*8 + C1];
  Arr1[i*64 + C2] += C3 * D;
  Arr1[i*64 + C2 + 2048] += C4 * D;
}

```
current default parameter value is not enough to run deeper cost analyze so the
loop is not completely unrolled.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D86248