For loads that operate on aggregate type, instcombine unpacks the loads.
It does not preserve the invariant.load metadata. This patch fixes that,
it looks for the metadata in the parent load and attaches the metadata
to the unpacked loads.
```
%struct.double2 = type { double, double }
%struct.double1 = type { double }
define %struct.double2 @func1(ptr %a) {
%1 = load %struct.double2, ptr %a, align 16, !invariant.load !1
ret %struct.double2 %1
}
!1 = !{}
```
Reproducer: https://godbolt.org/z/hcY8MMvYh
44df9826f3