freedreno/a3xx: fix for unused inputs

An unused input might not have a register assigned.  We don't want bogus
regid to result in impossibly high max_reg..
WIP: freedreno/lowering: almost starting two-sided-color
HACK: add test harness and simulator/emulator

Feed in TGSI, and it compiles with both old and new compiler, and runs
resulting shader through emulator with same set of randomized inputs.  A
convenient way to test the new compiler.

Long term, maybe break the simulator out into a stand alone thing (which
takes ir3 asm as input??), since it is probably useful to keep even
after old compiler is removed.
squash? MAYBE? HACK: add test harness and simulator/emulator
squash! HACK: add test harness and simulator/emulator
squash! HACK: add test harness and simulator/emulator
WIP: freedreno/lowering: start adding two-sided color emulation support..