glsl: Add ir_triop_vector_insert

glsl: Add ir_triop_vector_insert

The new opcode is used to generate a new vector with a single field from
the source vector replaced.  This will eventually replace
ir_dereference_array of vectors in the LHS of assignments.

v2: Convert tabs to spaces.  Suggested by Eric.

v3: Add constant expression handling for ir_triop_vector_insert.  This
prevents the constant matrix inversion tests from regressing.  Duh.

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