nir/spirv/cfg: Only set fall to true at the start of a case

nir/spirv/cfg: Only set fall to true at the start of a case

Previously, we were setting it to true at the top of the switch statement.
However, this causes all of the cases to get executed until you hit a
break.  Instead, you want to be not executing at the start, start executing
when you hit your case, and end at a break.