[DAGCombine][NFC] GatherAllAliases should take a LSBaseSDNode.
[DAGCombine][NFC] GatherAllAliases should take a LSBaseSDNode.

GatherAllAliases only makes sense for LSBaseSDNode. Enforce it with
static typing instead of runtime cast.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@353291 91177308-0d34-0410-b5e6-96231b3b80d8

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