[MachineScheduler] Add support for clustering mem ops with FI base operands

Before this patch, the following stores in `merge_fail` would fail to be
merged, while they would get merged in `merge_ok`:

```
void use(unsigned long long *);
void merge_fail(unsigned key, unsigned index)
{
  unsigned long long args[8];
  args[0] = key;
  args[1] = index;
  use(args);
}
void merge_ok(unsigned long long *dst, unsigned a, unsigned b)
{
  dst[0] = a;
  dst[1] = b;
}
```

The reason is that `getMemOpBaseImmOfs` would return false for FI base
operands.

This adds support for this.

Differential Revision: https://reviews.llvm.org/D54847

llvm-svn: 347747