Reapply [SCCP] Constant propagation through freeze instruction

Reapply with extra check for struct types, which caused buildbot
failures last time.

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The freeze instruction has not been handled by SCCPInstVisitor.
This patch adds SCCPInstVisitor::visitFreezeInst(FreezeInst &I)
method to handle freeze instructions.

Differential Revision: https://reviews.llvm.org/D151659