glsl: Set proper swizzle when a channel is missing in vectorizing.

glsl: Set proper swizzle when a channel is missing in vectorizing.

Previously, for example if the x channel was missing from a series of
assignments we were attempting to vectorize, the wrong swizzle mask
would be applied.

   a.y = b.y;
   a.z = b.z;
   a.w = b.w;

would be incorrectly transformed into

   a.yzw = b.xyz;

Fixes two transform feedback tests in the ES3 conformance suite.
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=73978
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=73954
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>