i965/vec4: check opcode on vec4_instruction::reads_flag(channel)

i965/vec4: check opcode on vec4_instruction::reads_flag(channel)

Commit f17b78 added an alternative reads_flag(channel) that returned
if the instruction was reading a specific channel flag. By mistake it
only took into account the predicate, but when the opcode is
VS_OPCODE_UNPACK_FLAGS_SIMD4X2 there isn't any predicate, but the flag
are used.

That mistake caused some regressions on old hw. More information on
this bug:
https://bugs.freedesktop.org/show_bug.cgi?id=92621

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>