[PATCH] Add a Scalarize pass

[Richard Sandiford]

  Thanks for the reviews.  New in this version:

  - The pass is called Scalarizer rather than DecomposeVectors.

  - Alignment is taken into account.

  - Only known metadata is kept.

  - Loads and stores are kept as-is if the elements are not a whole number of bytes in size.  It's still useful to decompose vectors of <N x i1> comparison results though -- in fact that was one of the original motivations.

  - Vector GEPs are scalarised too.  (I hadn't realised they were allowed, thanks.)

  - There are more tests.  Let me know if there's something else I should be testing for though.

  I tried adapting the llvm-stress wrapper in:

      http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-March/047876.html

  to run opt at -O3 with -enable-scalarizer -scalarize-load-store.  The number of runs and instruction sizes were both 1000.  No problems were reported.

  The pass doesn't yet try to scalarise function calls.  At the moment we just gather/reassemble vectors whenever we have a use that isn't understood.  That's enough for my target but it could be extended in future.  (In the SystemZ llvmpipe code that I'm working with, the only function call is to sqrt.v4f32, but these testcases wouldn't benefit by scalarising that early.)

http://llvm-reviews.chandlerc.com/D2112

CHANGE SINCE LAST DIFF
  http://llvm-reviews.chandlerc.com/D2112?vs=5376&id=5399#toc

Files:
  include/llvm-c/Transforms/Scalar.h
  include/llvm/Analysis/TargetTransformInfo.h
  include/llvm/InitializePasses.h
  include/llvm/LinkAllPasses.h
  include/llvm/Transforms/Scalar.h
  lib/Analysis/TargetTransformInfo.cpp
  lib/Transforms/IPO/PassManagerBuilder.cpp
  lib/Transforms/Scalar/CMakeLists.txt
  lib/Transforms/Scalar/Scalar.cpp
  lib/Transforms/Scalar/Scalarizer.cpp
  test/Transforms/Scalarizer/basic.ll
  test/Transforms/Scalarizer/dbginfo.ll
  test/Transforms/Scalarizer/no-data-layout.ll
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