Switch to llvm::findFirstSet instead of ffs since mingw doesn't have ffs.

Switch to llvm::findFirstSet instead of ffs since mingw doesn't have ffs.

ffs() vs findFirstSet() are slightly different, indexing is
0-based instead of 1-based.

Example mingw error:
http://build.chromium.org/p/tryserver.nacl/builders/nacl-toolchain-win7-pnacl-x86_64/builds/1920/steps/llvm_i686_w64_mingw32%20%28build%29/logs/stdio

BUG=none
R=stichnot@chromium.org

Committed: https://gerrit.chromium.org/gerrit/gitweb?p=native_client/pnacl-subzero.git;a=commit;h=1eb3a55dd9a08ceffe545b144a3be9ba63955035

[jvoung (off chromium), 2014-09-12 17:43:33]

jvoung@chromium.org changed reviewers:
+ stichnot@chromium.org

[Jim Stichnoth, 2014-09-12 17:49:22]

lgtm

[jvoung (off chromium), 2014-09-12 18:05:51]

Committed patchset #1 (id:1) manually as 1eb3a55 (presubmit successful).