[MDA] change BlockScanLimit into a command line option.

[MDA] change BlockScanLimit into a command line option.

Summary:
In the benchmark (https://github.com/vetter/shoc) we are researching,
the duplicated load is not eliminated because MemoryDependenceAnalysis
hit the BlockScanLimit. This patch change it into a command line option
instead of a hardcoded value.

Patched by Xuetian Weng.

Test Plan: test/Analysis/MemoryDependenceAnalysis/memdep-block-scan-limit.ll

Reviewers: jingyue, reames

Subscribers: reames, llvm-commits

Differential Revision: http://reviews.llvm.org/D11366

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242842 91177308-0d34-0410-b5e6-96231b3b80d8