[LV] Refactor ILV to provide vectorizeInstruction(); NFC
[LV] Refactor ILV to provide vectorizeInstruction(); NFC

Refactoring InnerLoopVectorizer's vectorizeBlockInLoop() to provide
vectorizeInstruction(). Aligning DeadInstructions with its only user.
Facilitates driving the transformation by VPlan - follows
https://reviews.llvm.org/D28975 and its tentative breakdown.

Differential Revision: https://reviews.llvm.org/D31997


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@300183 91177308-0d34-0410-b5e6-96231b3b80d8

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