[MachineSink] Fix stable sort comparator

[ellishg]

Fix the comparator in stable_sort() to satisfy the strict weak ordering requirement.

In #115367 this comparator was changed to use getCycleDepth() when shouldOptimizeForSize() is true. However, I mistakenly changed to logic so that we use LHSFreq < RHSFreq if either of them are zero. This causes us to fail the last requirment (https://en.cppreference.com/w/cpp/named_req/Compare).

if comp(a, b) == true and comp(b, c) == true then comp(a, c) == true

[llvm-ci]

LLVM Buildbot has detected a new failure on builder openmp-offload-libc-amdgpu-runtime running on omp-vega20-1 while building llvm at step 10 "Add check check-offload".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/73/builds/8832

Here is the relevant piece of the build log for the reference

Step 10 (Add check check-offload) failure: 1200 seconds without output running [b'ninja', b'-j 32', b'check-offload'], attempting to kill
...
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/bug53727.cpp (947 of 960)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/bug47654.cpp (948 of 960)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/bug50022.cpp (949 of 960)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/test_libc.cpp (950 of 960)
PASS: libomptarget :: x86_64-unknown-linux-gnu :: offloading/bug49021.cpp (951 of 960)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/wtime.c (952 of 960)
PASS: libomptarget :: x86_64-unknown-linux-gnu :: offloading/std_complex_arithmetic.cpp (953 of 960)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/complex_reduction.cpp (954 of 960)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/bug49021.cpp (955 of 960)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/std_complex_arithmetic.cpp (956 of 960)
command timed out: 1200 seconds without output running [b'ninja', b'-j 32', b'check-offload'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1237.835988