This effectively does the opposite of nir_lower_alus_to_scalar, trying
to combine per-component ALU operations with the same sources but
different swizzles into one larger ALU operation. It uses a similar
model as CSE, where we do a depth-first approach and keep around a hash
set of instructions to be combined, but there are a few major
differences:

1. For now, we only support entirely per-component ALU operations.
2. Since it's not always guaranteed that we'll be able to combine
equivalent instructions, we keep a stack of equivalent instructions
around, trying to combine new instructions with instructions on the
stack.

The pass isn't comprehensive by far; it can't handle operations where
some of the sources are per-component and others aren't, and it can't
handle phi nodes. But it should handle the more common cases, and it
should be reasonably efficient.

[Alyssa: Rebase on latest master, updating with respect to typeless
moves]

Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>