[PowerPC] Support basic compare mnemonics

This adds support for the basic mnemoics (with the L operand) for the
fixed-point compare instructions.  These are defined as aliases for the
already existing CMPW/CMPD patterns, depending on the value of L.

This requires use of InstAlias patterns with immediate literal operands.
To make this work, we need two further changes:

 - define a RegisterPrefix, because otherwise literals 0 and 1 would
   be parsed as literal register names

 - provide a PPCAsmParser::validateTargetOperandClass routine to
   recognize immediate literals (like ARM does)