CodeGen: BlockPlacement: Increase tail duplication size for O3.

At O3 we are more willing to increase size if we believe it will improve
performance. The current threshold for tail-duplication of 2 instructions is
conservative, and can be relaxed at O3.

Benchmark results:
llvm test-suite:
6% improvement in aha, due to duplication of loop latch
3% improvement in hexxagon

2% slowdown in lpbench. Seems related, but couldn't completely diagnose.

Internal google benchmark:
Produces 4% improvement on internal google protocol buffer serialization
benchmarks.

Differential-Revision: https://reviews.llvm.org/D32324
llvm-svn: 303084