Make disassembly of FC* more precise.

FCADD and FCMLA both span multiple opcodes, so it is not easy to represent them
with the Fixed/FMask/Mask structure that we usually use. In particular, the
Disassembler and Simulator would handle some unallocated encodings as FCADD
instructions.

This patch makes disassembly precise.

Change-Id: Ia0bda19489b2a1744b52a717ee7d6dc1b309184d