nvc0/ir: Allow 0/1 immediate value as source of OP_VOTE

nvc0/ir: Allow 0/1 immediate value as source of OP_VOTE

Implementation of readFirstInvocationARB() on nvidia hardware needs a
ballotARB(true) used to decide the first active thread. This expressed
in gm107 asm as (supposing output is $r0):
	vote any $r0 0x1 0x1

To model the always true input, which corresponds to the second 0x1
above, we make OP_VOTE accept immediate value 0/1 and emit "0x1" and
"not 0x1" in the src field respectively.

v2: Make sure that asImm() is not NULL (Samuel Pitoiset)

v3: (Ilia Mirkin)
Make the handling more symmetric with predicate version in gm107
Use i->getSrc(s)

Signed-off-by: Boyan Ding <boyan.j.ding@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>