nv50/ir: check for hasJoin before propagating joins
given this part of an shader the bra BB:7 in B:5 would get replaced by join even if the target doesn't support jumping joins.
BB:5
54: mov u32 $r1 0x00000000
55: bra BB:7 <== join from BB:7 would get propagated
BB:6
56: set ftz u8 $p0 neu f32 $r1 $r255
57: selp u32 $r1 $r255 0xffffffff not $p0
58: bra BB:7
BB:7
59: join
BB:8
Fixes: 5959d424 ("nv50/ir/opt: Fix tryPropagateBranch for BBs with several exit branches.")