[PowerPC] Enable printing instructions using aliases

TableGen had been nicely generating code to print a number of instructions using
shorter aliases (and PowerPC has plenty of short mnemonics), but we were not
calling it. For some of the aliases we support in the parser, TableGen can't
infer the "inverse" alias relationship, so there is still more to do.

Thus, after some hours of updating test cases...

llvm-svn: 235616