i#5070: AArch64 decode: fixes codec.py to handle overlapping encodings

This patch fixes the inability of codec.py to generate decode logic for certain combinations of encoding definitions.

Specifically, cases where the ONLY difference between two encoding definitions is some part of an operand field and some part of an opcode field. Examples are:

0x00111100000xxx111111xxxxxxxxxx fmov dq0 : fpimm8 h_sz
0x0011110xxxxxxx111111xxxxxxxxxx fcvtzs dq0 : dq5 sd_sz immhb

and

0x00111100000xxxxxxx01xxxxxxxxxx movi dq0 : imm8 cmode4
0x00111100000xxxxxx101xxxxxxxxxx orr dq0 : imm8 cmode3

Issues: #5070 (closed), #2626