i965/vs: Fix swizzle related assertion

i965/vs: Fix swizzle related assertion

As innocuous as it seemed, ebca47a basically broke the world (e.g.,
>200 piglit regressions).  In vec4_visitor::emit_block_move,
src->swizzle was expected to be BRW_SWIZZLE_NOOP before setting it to
a swizzle that would replicate the existing channels of the source
type to a vec4 (e.g., .xyyy for a vec2).

The original assertion seems to have been a little bogus.  In addition
to being BRW_SWIZZLE_NOOP, src->swizzle might already be a swizzle
that would replicate the existing channels of the source type to a
vec4.  In other words, it might already have the value that we're
about to assign to it.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>