prog_optimize: Set unused regs to PROGRAM_UNDEFINED after CMP->MOV conversion

prog_optimize: Set unused regs to PROGRAM_UNDEFINED after CMP->MOV conversion

Leaving the unused registers with other values caused assertion
failures and other problems in places that blindly iterate over all
sources.

brw_vs_emit.c:1381: get_src_reg: Assertion `c->regs[file][index].nr !=
0' failed.

Fixes i965 piglit:

    vs-uniform-array-mat[234]-col-row-rd
    vs-uniform-array-mat[234]-index-col-row-rd
    vs-uniform-array-mat[234]-index-row-rd
    vs-uniform-mat[234]-col-row-rd

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>