JIT: Fix invalid IR and weights in if-conversion

[BoyBaykiller]

Fix #124942.

Small diffs are expected because:

1. It allows if-conversion to recognize more cases.
   For example https://godbolt.org/z/Pdj898qTT currently still produces one jmp. But with this change it's eliminated, as the search now knows when there actually is a return block (previously it wasn't marked as such).

2. The block weights changed. For example previously

static int InvalidIR(bool cond)
{
    if (cond)
    {
        return 2;
    }
    // or else {}
    return 4;
}

produced blocks with bbWeight = 0.5, now it's 1.0.

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