[AMDGPU][CostModel] Refine cost model for control-flow instructions.

Added cost estimation for switch instruction, updated costs of branches, fixed
phi cost.
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
`PPCTTIImpl::getCFInstrCost()`.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D96805