i965/vec4: Add ability to reswizzle arbitrary swizzles.

i965/vec4: Add ability to reswizzle arbitrary swizzles.

Before commit 04895f5c we would only reswizzle dot product instructions
(since they wrote the same value into all channels, and we didn't have
to think about anything else). That commit extended reswizzling to cases
when the swizzle was single valued -- i.e., writing the same result into
all channels.

But allowing reswizzling of arbitrary things is actually really easy and
is even less code. (Why didn't we do this in the first place?!)

total instructions in shared programs: 4266079 -> 4261000 (-0.12%)
instructions in affected programs:     351933 -> 346854 (-1.44%)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>