[CIR][Lowering] Fix BrCond Lowering

[bruteforceboy]

This PR fixes the lowering for BrCond.

Consider the following code snippet:

#include <stdbool.h>

bool test() {
  bool x = false;
  if (x) 
    return x;
  return x;
} 

Emitting the CIR to tmp.cir using -fclangir-mem2reg produces the following CIR (truncated):

!s32i = !cir.int<s, 32>
#fn_attr = #cir<extra({inline = #cir.inline<no>, nothrow = #cir.nothrow, optnone = #cir.optnone})>
module { cir.func no_proto  @test() -> !cir.bool extra(#fn_attr) {
    %0 = cir.const #cir.int<0> : !s32i 
    %1 = cir.cast(int_to_bool, %0 : !s32i), !cir.bool 
    cir.br ^bb1 
  ^bb1:  // pred: ^bb0
    cir.brcond %1 ^bb2, ^bb3 
  ^bb2:  // pred: ^bb1
    cir.return %1 : !cir.bool 
  ^bb3:  // pred: ^bb1
    cir.br ^bb4 
  ^bb4:  // pred: ^bb3
    cir.return %1 : !cir.bool 
  } 
}

Lowering the CIR to LLVM using cir-opt tmp.cir -cir-to-llvm fails with:

tmp.cir:5:10: error: failed to legalize operation 'llvm.zext' marked as erased

The CIR cast %1 = cir.cast(int_to_bool, %0 : !s32i) is lowered to a CIR comparison with zero, which is then lowered to an LLVM::ICmpOp and LLVM::ZExtOp.

In the BrCond lowering, the zext is deleted when zext->use_empty(), but during this phase the lowering for the CIR above is not complete yet, because the zext will still have usage(s) later.

The current check for when the zext is deleted is error-prone and can be improved.

To fix this, in addition to checking that the use of the zext is empty, an additional check that the defining operation for the BrCond in the CIR (the cast operation in this case) is used exactly once is added.

[bcardosolopes]

Thanks for fixing this bug and thanks @smeenai for reviewing! Some changes needed but this almost good to go.