Allow lowering if-else from LLVM dialect to LLVMIR

[rikhuijzer]

Allows lowering

module {
  llvm.func @main() -> i32 {
    %false = llvm.mlir.constant(false) : i1
    llvm.cond_br %false, ^then, ^else
  ^then:
    %c3_i32 = llvm.mlir.constant(3 : i32) : i32
    llvm.br ^merge(%c3_i32 : i32)
  ^else:
    %c4_i32 = llvm.mlir.constant(4 : i32) : i32
    llvm.br ^merge(%c4_i32 : i32)
  ^merge(%result : i32):
    llvm.br ^exit
  ^exit:
    llvm.return %result : i32
  }
}

to

; ModuleID = 'LLVMDialectModule'
source_filename = "LLVMDialectModule"

define i32 @main() {
  br i1 false, label %1, label %2

1:                                                
  br label %3

2:                                               
  br label %3

3:                                              
  %result = phi i32 [ 4, %2 ], [ 3, %1 ]
  br label %5

5:                                             
  ret i32 %result
}

!llvm.module.flags = !{!0}

!0 = !{i32 2, !"Debug Info Version", i32 3}

which is the same result as mlir-opt --convert-mlir-to-llvmir (functionality-wise at least, MLIR provides some extra debugging info as comments and renames %result to %4).