HLSL holds the compare value in a separate intrinsic arg, but the AST wants
a vector including the cmp val, except in the 4-dim coord case, where it
doesn't fit and is in fact a separate AST parameter.  This is awkward but
necessary, given AST semantics.  In the process, a new vector is constructed
for the combined result, but this vector was not being given the correct
TType, so was causing some downstream troubles.

Now it is.  A similar defect existed in OpTextureBias, and has also been
fixed.