[LoopStrengthReduce, x86] don't add cost for a cmp that will be macro-fused (PR35681)

In the motivating case from PR35681 and represented by the macro-fuse-cmp test:
https://bugs.llvm.org/show_bug.cgi?id=35681
...there's a 37 -> 31 byte size win for the loop because we eliminate the big base 
address offsets.

SPEC2017 on Ryzen shows no significant perf difference.

Differential Revision: https://reviews.llvm.org/D42607

llvm-svn: 324289