rs6000: Merge the var_shift yes/no alternatives

rs6000: Merge the var_shift yes/no alternatives

All instructions that are "var_shift" for some alternative have the shift
amount as operands[2].

This patch introduces an attribute "maybe_var_shift".  If that is set to
"yes", the default value of "var_shift" is set based on the operands[2]
value.

With that, we can merge the var_shift yes/no cases everywhere.  Do so.

Also change some more "i" to "n".

From-SVN: r211880