.../asm-i386/bitops.h performance improvements

In find_first_bit() there exists this the sequence:

shll $3,%%edi
addl %%edi,%%eax

LEA knows how to multiply by small powers of 2 and add all in one shot very
efficiently:

leal (%%eax,%%edi,8),%%eax


In find_first_zero_bit() the sequence:

shll $3,%%edi
addl %%edi,%%edx

could similarly become:

leal (%%edx,%%edi,8),%%edx



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