[TwoAddressInstructionPass] Try 3 Addr Conversion After Commuting.

TwoAddressInstructionPass stops after a successful commuting but 3 Addr
conversion might be good for some cases.
 
Consider:

int foo(int a, int b) {
  return a + b;
}

Before this commit, we emit:

addl	%esi, %edi
movl	%edi, %eax
ret

After this commit, we try 3 Addr conversion:

leal	(%rsi,%rdi), %eax
ret

Patch by Volkan Keles <vkeles@apple.com>!

Differential Revision: http://reviews.llvm.org/D10851

llvm-svn: 241206