PPC64: Fix load store instructions

PPC64: Fix load store instructions

This patch fixes numerous errors in the ISA support.

The word and prefix versions of the instructions do not use the same mask
to extract the immediate values.  The prefix instructions should all use
the DFOM_IMMASK.

The parsing of prefix instructions has been fixed to ensure the ISA 3.1
instructions all have the ISA_3_1_PREFIX_CHECK check.

Fixed the commenting to improve the comments for the instruction parsing.

Fixed the parsing of the plxv instruction.

General code cleanup.