[DAGCombiner] Reapply load slicing (192471) with a test that explicitly set sse4.2 support.
This should fix the buildbots.

Original commit message:
[DAGCombiner] Slice a big load in two loads when the element are next to each
other in memory and the target has paired load and performs post-isel loads
combining.

E.g., this optimization will transform something like this:
a = load i64* addr
b = trunc i64 a to i32
c = lshr i64 a, 32
d = trunc i64 c to i32

into:
b = load i32* addr1
d = load i32* addr2
Where addr1 = addr2 +/- sizeof(i32), if the target supports paired load and
performs post-isel loads combining.

One should overload TargetLowering::hasPairedLoad to provide this information.
The default is false.

<rdar://problem/14477220>