[SandboxIR] Implement UncondBrInst and CondBrInst (#187196)
This patch implements the unconditional and conditional branch
instructions mirroring the newly added LLVM IR instructions.
So now we have two new classes UncondBrInst and CondBrInst inheriting
from BranchInst.
The original Br opcode has been removed in favor of UncondBr and CondBr.