More readable trapped rook condition

Prefer

file_of(s) < file_of(ksq)

to the inidrect

file_of(ksq) < FILE_E

To evaluate if semiopen side to check is the left side.

Also other small touches while there.

No functional change.
Merge remote-tracking branch 'upstream/master'
Enable BMI1 too when using BMI2 ARCH

Adding BMI1 allows the compiler to use _blsr_u64
automatically (the advertised 0.3% speed gain).
I verified that the compiler does not use this
instruction with the -mbmi2 flag only. Also, all
processors supporting BMI2 is also supporting BMI1.

No functional change
Merge remote-tracking branch 'upstream/master'