alive2 backend needs to understand phi

[regehr]

this is for @manasij7479 and @zhengyangl

alive2 does not understand phi nodes, so it cannot be used to double-check a synthesis result such as this one

regehr@home:~$ cat nop7.opt

; RUN: %souper-check -infer-rhs %s > %t1
; RUN: %FileCheck %s < %t1

; CHECK: Failed to infer RHS

%0 = block 5
%1:i1 = var
%2:i1 = var
%3:i1 = or %1, %2
%4:i1 = phi %0, %1, %1, %1, %1, %1
infer %4
regehr@home:~$ ~/souper-regehr/build/souper-check -infer-rhs nop7.opt 
Dataflow Pruned 0/0
There are 2 Guesses
; RHS inferred successfully
result %1
regehr@home:~$ ~/souper-regehr/build/souper-check -infer-rhs nop7.opt -souper-double-check
souper-check: ../lib/Infer/AliveDriver.cpp:628: bool souper::AliveDriver::translateAndCache(const souper::Inst *, IR::Function &, souper::AliveDriver::Cache &): Assertion `false && "Phi with muliple arguments unimplemented"' failed.
Aborted (core dumped)
regehr@home:~$ 

[manasij7479]

Implemented