Preserve OSIZE for APX EVEX MAP4

[RatinCN]

Addendum to #374, already fixed in my SlimDetours in commit da1e214.

EVEX MAP4 promotes legacy integer instructions, so EVEX.pp=01 can still act as an operand-size override when W=0. Keep mandatory-prefix fixed sizing for other VEX/EVEX maps, but allow MAP4 to select nFixedSize16.

#374 made all VEX/EVEX instructions use nFixedSize, ignoring m_bOperandOverride.
That is correct for ordinary VEX/EVEX maps where pp is a mandatory prefix, but APX EVEX MAP4 promotes legacy integer instructions.
For MAP4, EVEX.pp=01 can still act as OSIZE when EVEX.W=0.

For example, an APX EVEX MAP4 Group 1 immediate instruction with pp=66, W=0, and opcode 81 /0 should consume an imm16. The current logic consumes imm32, over-reading two bytes from the following instruction.

62 04 05 00	EVEX.66.MAP4.W0
81 C0		Group 1 /0, register form
34 12		imm16

After applying this PR, this instruction will be calculated as 8 bytes in size, no longer 10 bytes.