[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)
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185826
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