This patch relaxes the conditions under which
we remove branch instructions. Additionally, it
introduces the use of VSKIP for GCN.

Removing the branches now happens especially
in these two situations:
- divergent breaks: the outer branch instruction can be removed
- optimized atomics: the branch condition is always true for one lane

Totals from 20846 (14.96% of 139391) affected shaders:
CodeSize: 156790560 -> 156294524 (-0.32%)
Instrs: 29870459 -> 29748936 (-0.41%)
Cycles: 1641405628 -> 1611934772 (-1.80%); split: -1.80%, +0.00%
VMEM: 4538623 -> 4536635 (-0.04%)
SMEM: 1318984 -> 1318787 (-0.01%)
Branches: 830286 -> 708791 (-14.63%)