Value stack underflow during JIT compilation (AB AB problem)

[kpp]

The problem is in MethodCompiler::TJITContext::popValue due to the linkage of Smalltalk stack with LLVM IR tree during JIT compilation.

Code to reproduce the bug:

Jit do: [ BranchTest new phi ].

METHOD BranchTest
phi |x|     
    x <- (true ifTrue:  [ 11 ]
               ifFalse: [ 17 ] ) +
         (true ifTrue:  [ 13 ]
               ifFalse: [ 15 ] ).

    [ x ] assertEq: 24.
!

Bytecode:

0000 PushConstant true
0001 DoSpecial branchIfFalse 8
0004 PushLiteral 0         ; SmallInt 11
0005 DoSpecial branch 9
0008 PushLiteral 1         ; SmallInt 17
0009 PushConstant true
0010 DoSpecial branchIfFalse 17
0013 PushLiteral 2         ; SmallInt 13
0014 DoSpecial branch 18
0017 PushLiteral 3         ; SmallInt 15
0018 SendBinary +
0019 AssignTemporary 0
0020 DoSpecial popTop
0021 PushBlock
0024     PushTemporary 0
0025     DoSpecial stackReturn
0026 PushLiteral 4         ; #assertEq:
0027 MarkArguments 2
0028 SendMessage assertEq:
0029 DoSpecial popTop
0030 DoSpecial selfReturn

The compiler links BBs together with "MethodCompiler::TRefererSet referers" in a linked tree:

A   B    A   B
 \ /      \ /
  C        D
   \      /
     \  /
       E

As you can see, there is a duplicate of AB part, which leads to stack underflow.
JIT compiler produces the following pseudocode:

A: push
B: push
C: push
D: push

E: pop -> phi C , D
E: pop -> phi -> C  pop -> phi A, B
                 D  pop -> Value stack underflow

The compiler should link BBs in the following way:

A   B    C   D
 \ /      \ /
  *        *
   \      /
     \  /
       E

E: pop -> phi A , B
E: pop -> phi C , D

[0x7CFE]

This bug is highly related to current implementation of MethodCompiler.

The upcoming instruction API in #32 will eliminate it automagically because of new graph representation of control flow.