[Refactor] Move VarUseDefAnalysis to header file

[yzh119]

Motivation

UndefinedVars is a frequently used function in our codebase and its implementation relies on VarUseDefAnalysis class which is more general, currently we expose UndefinedVars in analysis.h, but both the definitions of UndefinedVars and VarUseDefAnalysis resides in split_host_device.cc.

This PR moves VarUseDefAnalysis class to analysis.h so that developers can use it in other files that requires use/def analysis than split_host_devices.cc. We create a var_use_def_analysis.cc under src/src/analysis for the implementations of both UndefinedVars and VarUseDefAnalysis.

Notes

We rename VarUseDefAnalysis to VarUseDefAnalyzer.

cc @Hzfengsy @Lunderberg

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[yzh119]

@Hzfengsy @Lunderberg @tqchen I have decoupled the previous VarDefUseAnalyzer class into three classes:

1. VarUseDefAnalyzer which is a StmtExprVisitor, we expose it to our files via a header under src
2. DeviceInfoCollector which is a StmtVisitor that collects program information about shared memory usage, thread axis/extents etc.
3. UnreferencedLetRemover which removes redundant let stmt/exprs.

I know this class has been used for long time and we don't want any changes on its behavior, so we need to be careful in this refactor, and please correct me if I made any mistakes. One thing I'm not sure is if we need to couple the three functionalities together in a single pass, or we can decouple them like what I did in the refactor.

If my refactor doesn't work, I'm totally okay if we keep the original class and create a simpler VarDefUseAnalyzer for light-weight analysis use (e.g. UndefinedVars).

[Lunderberg]

LGTM! Just a couple of small nitpicks.

[tqchen]

cc @Hzfengsy please take another look and manage the PR