[MemDep] allow to select block-scan-limit when constructing MemoryDependenceAnalysis

Introducing non-global control for default block-scan-limit in MemDep analysis.
Useful when there are many compilations per initialized LLVM instance (e.g. JIT).

Reviewed By: asbirlea
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D65806

llvm-svn: 368502