radv/rt: Use nested ifs for rt cases

Quake II RTX:

74fps -> 78fps

Totals from 7 (14.00% of 50) affected shaders:
CodeSize: 216936 -> 218464 (+0.70%)
Instrs: 40318 -> 40633 (+0.78%)
Latency: 933064 -> 619794 (-33.57%)
InvThroughput: 189384 -> 125368 (-33.80%)
Copies: 5608 -> 5674 (+1.18%); split: -0.14%, +1.32%
Branches: 1933 -> 2020 (+4.50%)
PreSGPRs: 383 -> 403 (+5.22%)
PreVGPRs: 642 -> 635 (-1.09%)

I haven't run CTS yet.