Up until now the generation of vector instructions was taking place
during the top-down post-order traversal of vectorizeRec(). The issue
with this approach is that the vector instructions emitted during the
traversal can be reordered by the scheduler, making it challenging to
place them without breaking the def-before-uses rule.

With this patch we separate the vectorization decisions (done in
`vectorizeRec()`) from the code generation phase (`emitVectors()`). The
vectorization decisions are stored in the `Actions` vector and are used
by `emitVectors()` to drive code generation.

This patch fixes a bug in the creation of shuffle masks when vectorizing
vectors in case of a diamond reuse with shuffle. The mask needs to
enumerate all elements of a vector, not treat the original vector value
as a single element. That is: if vectorizing two <2 x float> vectors
into a <4 x float> the mask needs to have 4 indices, not just 2.

This patch fixes:

  third-party/unittest/googletest/include/gtest/gtest.h:1379:11:
  error: comparison of integers of different signs: 'const unsigned
  int' and 'const int' [-Werror,-Wsign-compare]

This patch hooks up InstrMaps to the Sandbox IR callbacks such that it
gets updated when instructions get erased.

This patch fixes:

  third-party/unittest/googletest/include/gtest/gtest.h:1379:11:
  error: comparison of integers of different signs: 'const unsigned
  int' and 'const int' [-Werror,-Wsign-compare]

  llvm/include/llvm/Transforms/Vectorize/SandboxVectorizer/InstrMaps.h:57:12:
  error: unused variable 'Pair' [-Werror,-Wunused-variable]

InstrMaps is a helper data structure that maps scalars to vectors and
the reverse. This is used by the vectorizer to figure out which vectors
it can extract scalar values from.