[llvm-mc] Ignore opcode size prefix in 64-bit CALL disassembly

This is a fix for disassembling unusual instruction sequences in 64-bit
mode w.r.t the CALL rel16 instruction. It might be desirable to move the
check somewhere else, but it essentially mimics the special case
handling with JCXZ in 16-bit mode.

The current behavior accepts the opcode size prefix and causes the
call's immediate to stop disassembling after 2 bytes. When debugging
sequences of instructions with this pattern, the disassembler output
becomes extremely unreliable and essentially useless (if you jump midway
into what lldb thinks is a unified instruction, you'll lose %rip). So we
ignore the prefix and consume all 4 bytes when disassembling a 64-bit
mode binary.

Note: in Vol. 2A 3-99 the Intel spec states that CALL rel16 is N.S. N.S.
is defined as:

    Indicates an instruction syntax that requires an address override
    prefix in 64-bit mode and is not supported. Using an address
    override prefix in 64-bit mode may result in model-specific
    execution behavior. (Vol. 2A 3-7)

Since 0x66 is an operand override prefix we should be OK (although we
may want to warn about 0x67 prefixes to 0xe8). On the CPUs I tested
with, they all ignore the 0x66 prefix in 64-bit mode.

Patch by Matthew Barney!

Differential Revision: http://reviews.llvm.org/D9573

llvm-svn: 246038