This patch updates/adds LLVM_DEBUG dumps.
It moves the DEBUG_TYPE into SandboxVectorizer/Debug.h such that it can
be shared across all components of the vectorizer.

When the operand comes from multiple inputs then we need additional
packing code. When the operands are scalar then we can use a single
InsertElementInst. But when the operands are vectors then we need a
chain of ExtractElementInst and InsertElementInst instructions to insert
the vector value into the destination vector. This is what this patch
implements.

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.

SelectInsts need special treatment because they are not always
straightforward to vectorize. This patch disables vectorization unless
they are trivially vectorizable.

This patch adds a legality check that checks for repeated instrs in a
bundle and won't vectorize if such pattern is found.

When the inputs of the pack come from different BBs we need to make sure
we emit the pack instructions at the correct place.

This patch implements the diamond pattern where we are vectorizing
toward the top of the diamond from both edges, but the second edge may
use elements from a different vector or just scalar values. This
requires some additional packing code (see lit test).

This patch implements a helper ShuffleMask data structure that helps
describe shuffles of elements across lanes.