[JumpThreading][LVI] Invalidate LVI information after blocks are merged

Summary:
After a single predecessor is merged into a basic block, we need to invalidate
the LVI information for the new merged block, when LVI is not provably true for
all of instructions in the new block.
The test cases added show the correct LVI information using the LVI printer
pass.

Reviewers: reames, dberlin, davide, sanjoy

Reviewed by: dberlin, davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34108

llvm-svn: 305699