ARB prog parser: use _mesa_add_unnamed_constant() to use fewer constant slots

ARB prog parser: use _mesa_add_unnamed_constant() to use fewer constant slots

This function will search the constant parameters in an effort to re-use
constant slots.  For example, {1,2,3,4} and {4,1,1,2} can be stored in
one constant slot and accessed with different swizzles.  The swizzle info
must be propogated though the parsing code in a few places.

Fixes Piglit "vpfp-generic tests/shaders/generic/big-param.vpfp" failure.