cmd/compile: enable CondSelect math for pow2 Or/Xor on amd64

[elee1766]

changes rewriteCondSelectIntoMath to also use inline register shifts
for amd64 for power of 2 cases, as opposed to the cmov based approach.
this is faster for the case of accumulating independent conditions
into a bitset, and slower for dependent conditions.

analysis of existing go code shows a 10:1+ ratio of independent to
dependent conditions. the independent condition is also found more
in hot paths where the optimization would help, for instance packing
headers, as opposed to the dependent pattern which is used far more
for reading flags for a first-time setup step.

addresses #80202 for the amd64 case

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[Jorropo]

Usually I would review on gerrit but it's 429ing me today.

[elee1766]

changed, and removed that really long commit details

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