PowerPC: Use RegisterOperand instead of RegisterClass operands

In the default PowerPC assembler syntax, registers are specified simply
by number, so they cannot be distinguished from immediate values (without
looking at the opcode).  This means that the default operand matching logic
for the asm parser does not work, and we need to specify custom matchers.
Since those can only be specified with RegisterOperand classes and not
directly on the RegisterClass, all instructions patterns used by the asm
parser need to use a RegisterOperand (instead of a RegisterClass) for
all their register operands.

This patch adds one RegisterOperand for each RegisterClass, using the
same name as the class, just in lower case, and updates all instruction
patterns to use RegisterOperand instead of RegisterClass operands.