[PATCH] [CodeGen] Add hooks/combine to form vector extloads, and enable it on X86.

[hfinkel at anl.gov]

================
Comment at: lib/CodeGen/SelectionDAG/DAGCombiner.cpp:5227
@@ +5226,3 @@
+
+  // fold (sext (load x)) -> (sext (truncate (sextload x)))
+  // Only on illegal but splittable vectors.
----------------
Please make this comment more verbose. We might generate multiple loads and then combine them together, and it is not clear here whether this combining happens to the results of the sext or the sext's operand.

================
Comment at: lib/CodeGen/SelectionDAG/DAGCombiner.cpp:5274
@@ +5273,3 @@
+      LN0->isVolatile(), LN0->isNonTemporal(), LN0->isInvariant(),
+      LN0->getAlignment(), LN0->getAAInfo());
+
----------------
Is this alignment correct? Aren't these loads smaller than the original one?

http://reviews.llvm.org/D6904

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