SLPVectorizer: When CSEing generated gathers only scan blocks containing them.
SLPVectorizer: When CSEing generated gathers only scan blocks containing them.

Instead of doing a RPO traversal of the whole function remember the blocks
containing gathers (typically <= 2) and scan them in dominator-first order.

The actual CSE is still quadratic, but I'm not confident that adding a
scoped hash table here is worth it as we're only looking at the generated
instructions and not arbitrary code.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@193956 91177308-0d34-0410-b5e6-96231b3b80d8

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