[PATCH][X86] Improved tablegen patters for matching TZCNT/LZCNT.

[Nadav Rotem]

LGTM!  Thanks Andrea. 

> On Dec 8, 2014, at 9:36 AM, Andrea Di Biagio <andrea.dibiagio at gmail.com> wrote:
> 
> Hi Nadav, Quentin (and all).
> 
> This patch teaches ISel how to match a TZCNT/LZCNT from a conditional
> move if the condition code is equal to X86_COND_NE.
> 
> Existing tablegen patterns only allowed to match a TZCNT/LZCNT from a
> X86cond with condition code equal to X86_COND_E. To avoid introducing
> extra rules, I added an 'ImmLeaf' definition to check if the condition
> code is COND_E or COND_NE.
> 
> Example:
> ///
> unsigned foo(unsigned A) {
>  unsigned bar = __builtin_clz(A);
>  return !A ? 32 : bar;
> }
> ///
> 
> Before, (on targets that feature LZCNT), we generated the following
> sub-optimal assembly:
>   lzcntl  %edi, %ecx
>   movl  $32, %eax
>   cmovael  %ecx, %eax
>   retq
> 
> Now we simply generate:
>   lzcntl  %edi, %eax
>   retq
> 
> Please let me know if ok to submit.
> 
> Thanks,
> Andrea
> <patch-lzcnt-tzcnt.diff>