Cross-class joining is winning.

Cross-class joining is winning.

Remove the heuristic for disabling cross-class joins. The greedy
register allocator can handle the narrow register classes, and when it
splits a live range, it can pick a larger register class.

Benchmarks were unaffected by this change.

<rdar://problem/11302212>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@155551 91177308-0d34-0410-b5e6-96231b3b80d8