[Codegen][X86] EltsFromConsecutiveLoads(): if only have AVX1, ensure that the...

[Codegen][X86] EltsFromConsecutiveLoads(): if only have AVX1, ensure that the "load" is actually foldable (PR51615)

This fixes another reproducer from https://bugs.llvm.org/show_bug.cgi?id=51615
And again, the fix lies not in the code added in D105390

In this case, we completely don't check that the "broadcast-from-mem" we create
can actually fold the load. In this case, it's operand was not a load at all:
```
Combining: t16: v8i32 = vector_shuffle<0,u,u,u,0,u,u,u> t14, undef:v8i32
Creating new node: t29: i32 = undef
RepeatLoad:
t8: i32 = truncate t7
  t7: i64 = extract_vector_elt t5, Constant:i64<0>
    t5: v2i64,ch = load<(load (s128) from %ir.arg)> t0, t2, undef:i64
      t2: i64,ch = CopyFromReg t0, Register:i64 %0
        t1: i64 = Register %0
      t4: i64 = undef
    t3: i64 = Constant<0>
Combining: t15: v8i32 = undef

```

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D108821