x86inc: Improve FMA instruction handling

x86inc: Improve FMA instruction handling

 * Correctly handle FMA instructions with memory operands.
 * Print a warning if FMA instructions are used without the correct cpuflag.
 * Simplify the instantiation code.
 * Clarify documentation.

Only the last operand in FMA3 instructions can be a memory operand. When
converting FMA4 instructions to FMA3 instructions we can utilize the fact
that multiply is a commutative operation and reorder operands if necessary
to ensure that a memory operand is used only as the last operand.

Signed-off-by: Anton Khirnov <anton@khirnov.net>