bmiintrin.h: Allow using the tzcnt intrinsics for non-BMI targets

The tzcnt intrinsics are used non non-BMI targets by code (e.g. ffmpeg)
that uses it as a potentially faster BSF.

The TZCNT instruction is special in that it's encoded in a
backward-compatible way and behaves as BSF on non-BMI targets.

Differential Revision: http://reviews.llvm.org/D14748

llvm-svn: 253358