[x86] Begin a significant overhaul of how vector lowering is done in the
x86 backend.

This sketches out a new code path for vector lowering, hidden behind an
off-by-default flag while it is under development. The fundamental idea
behind the new code path is to aggressively break down the problem space
in ways that ease selecting the odd set of instructions available on
x86, and carefully avoid scalarizing code even when forced to use older
ISAs. Notably, this starts off restricting itself to SSE2 and implements
the complete vector shuffle and blend space for 128-bit vectors in SSE2
without scalarizing. The plan is to layer on top of this ISA extensions
where we can bail out of the complex SSE2 lowering and opt for
a cheaper, specialized instruction (or set of instructions). It also
needs to be generalized to AVX and AVX512 vector widths.

Currently, this does a decent but not perfect job for SSE2. There are
some specific shortcomings that I plan to address:
- We need a peephole combine to fold together shuffles where possible.
  There are cases where a previous shuffle could be modified slightly to
  arrange for elements to be in the correct position and a later shuffle
  eliminated. Doing this eagerly added quite a bit of complexity, and
  so my plan is to combine away these redundancies afterward.
- There are a lot more clever ways to use unpck and pack that need to be
  added. This is essential for real world shuffles as it turns out...

Once SSE2 is polished a bit I should be able to get interesting numbers
on performance improvements on benchmarks conducive to vectorization.
All of this will be off by default until it is functionally equivalent
of course.

Differential Revision: http://reviews.llvm.org/D4225