[AMDGPU][CostModel] Refine cost model for control-flow instructions.
Added cost estimation for switch instruction, updated costs of branches, fixed
Had to increase -amdgpu-unroll-threshold-if default value since conditional
branch cost (size) was corrected to higher value.
Test renamed to "control-flow.ll".
Removed redundant code in X86TTIImpl::getCFInstrCost() and
Reviewed By: rampitec
Differential Revision: https://reviews.llvm.org/D96805