[SROA] Create additional vector type candidates based on store and load slices

This patch adds additional vector types to be considered when doing
promotion in SROA, based on the types of the store and load slices. This
provides more promotion opportunities, by potentially using an optimal
"intermediate" vector type.

For example, the following code would currently not be promoted to a
vector, since `__m128i` is a `<2 x i64>` vector.

```

__m128i packfoo0(int a, int b, int c, int d) {
  int r[4] = {a, b, c, d};
  __m128i rm;
  std::memcpy(&rm, r, sizeof(rm));
  return rm;
}
```

```
packfoo0(int, int, int, int):
        mov     dword ptr [rsp - 24], edi
        mov     dword ptr [rsp - 20], esi
        mov     dword ptr [rsp - 16], edx
        mov     dword ptr [rsp - 12], ecx
        movaps  xmm0, xmmword ptr [rsp - 24]
        ret
```

By also considering the types of the elements, we could find that the
`<4 x i32>` type would be valid for promotion, hence removing the memory
accesses for this function. In other words, we can explore other new
vector types, with the same size but different element types based on
the load and store instructions from the Slices, which can provide us
more promotion opportunities.

Additionally, the step for removing duplicate elements from the
`CandidateTys` vector was not using an equality comparator, which has
been fixed.

Differential Revision: https://reviews.llvm.org/D132096