[X86] Update test to not iterate past array boundaries. (#163991)

This test has loop iterating past (`61`) the array boundaries (`58`). So
far this didn't seem to matter, but recently with this change
https://github.com/llvm/llvm-project/pull/155253 the constraint
elimination in swift has been able to figure this out and is
transforming the loop into an infinite one like this
```
*** IR Dump After ConstraintEliminationPass on test_known_trip_count ***
define void @test_known_trip_count() local_unnamed_addr {
entry:
  br label %for.body

for.body:                                         ; preds = %entry, %for.body
  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
  %arrayidx = getelementptr inbounds nuw double, ptr @b, i64 %indvars.iv
  %0 = load double, ptr %arrayidx, align 8
  %arrayidx2 = getelementptr inbounds nuw double, ptr @c, i64 %indvars.iv
  %1 = load double, ptr %arrayidx2, align 8
  %add = fadd double %0, %1
  %arrayidx4 = getelementptr inbounds nuw double, ptr @a, i64 %indvars.iv
  store double %add, ptr %arrayidx4, align 8
  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
  br i1 false, label %exit, label %for.body

exit:                                             ; preds = %for.body
  ret void
}%
```
causing the test to fail. This is trying to address the root cause here.