(venom): convert selection branches to branchless select

[charles-cooper]

for branches which just serve the purpose of selecting a variable for a phi, we should be able to convert to a branchless select. for instance, the following code

main:
    %1 = ...
    %2 = ...
    %cond = ...
    jnz %cond, @then, @else

then:
    %3 = %1
    jmp @join

else:
    %4 = %2
    jmp @join

join:
    %5 = phi @then, %3, @else, %4

could be converted to

main:
    %1 = ...
    %2 = ...
    %cond = ...
    %tmp1 = xor %1, %2
    %tmp2 = iszero %cond  ; ensure it's in (0, 1)
    %tmp3 = mul %tmp2, %tmp1
    %tmp4 = xor %tmp3, %1  ; or maybe xor %tmp3, %2? double check
    %5 = %tmp4

[charles-cooper]

i've made a first pass at this at https://github.com/vyperlang/vyper/compare/master...charles-cooper:vyper:feat/venom/branch-xor-reducer?expand=1