CPU chip: failure to satisfy LOADFP output constraint

[morganthomas]

On an example in the Valida LLVM compiler test suite, the trace fails to satisfy a particular constraint of the CPU chip, in cpu/src/stark.rs:

builder
    .when(is_loadfp)
    .assert_eq(local.fp, reduce::<AB>(base, local.write_value()));

This file compiles into the following assembly code:

IMM32 -8, 0, 0, 0, 0
JAL -4, 1e0, 0, 1, 1
IMM32 -8, 0, 0, 0, 14
JAL -10, 90, -14, 1, 1
STOP 0, 0, 0, 0, 0
JALV -4, 4, c, 0, 0
IMM32 -10, 0, 0, 0, 0
ADD32 -4, -10, 0, 0, 1
IMM32 -14, 0, 0, 0, 5
ADD32 -8, -14, 0, 0, 1
a -18, -8, 0, 1, 1
ADD32 -c, -18, 0, 0, 1
ADD32 -1c, -c, 0, 0, 1
BEQ 198, -1c, 0, 0, 1
BEQ 168, 4, 4, 0, 0
BEQ 180, 4, 4, 0, 0
BEQ 180, 4, 4, 0, 0
IMM32 -20, 0, 0, 0, 0
ADD32 8, -20, 0, 0, 1
JALV -4, 4, c, 0, 0
IMM32 4, 0, 0, 0, 0
JALV 0, -4, -8, 0, 0

The following is the result of trying to prove an execution of this program:

Proof successful
thread 'main' panicked at /home/morgan/code/lita/issue/completeness/valida/basic/src/lib.rs:54:10:
Failed to verify constraints on chip 0: OodEvaluationMismatch
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

When the mentioned constraint is commented out, then proving succeeds and results in a verifiable proof of execution for this example.

Produce a minimal example which exhibits the issue. Fix the issue. Add a regression test to the test suite which verifies the proof of an execution of a minimal example which exhibits the issue.