The patch adds CTLZ idiom recognition.

Summary:

The following loops should be recognized:
i = 0;
while (n) {
  n = n >> 1;
  i++;
  body();
}
use(i);

And replaced with builtin_ctlz(n) if body() is empty or
for CPUs that have CTLZ instruction converted to countable:

for (j = 0; j < builtin_ctlz(n); j++) {
  n = n >> 1;
  i++;
  body();
}
use(builtin_ctlz(n));

Reviewers: rengolin, joerg

Differential Revision: http://reviews.llvm.org/D32605

From: Evgeny Stupachenko <evstupac@gmail.com>
llvm-svn: 303102