[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>

llvm-svn: 192471

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