ilog2: optimize use of bsr for x86-64
ilog2: optimize use of bsr for x86-64

On x86-64 platforms, we can rely on BSR not changing the destination
operand when the input is zero.

Signed-off-by: H. Peter Anvin <8a453bad9912ffe59bc0f0b8abe03df9be19379e@zytor.com>

techkey/nasm,Distrotech/nasm,projedi/nasm,Distrotech/nasm,letolabs/nasm,projedi/nasm,Distrotech/nasm,letolabs/nasm,techkey/nasm,techkey/nasm,Distrotech/nasm,techkey/nasm,turingstudio/nasm,projedi/nasm,turingstudio/nasm,projedi/nasm,techkey/nasm,turingstudio/nasm,projedi/nasm,turingstudio/nasm,letolabs/nasm