x86: fix near RET decoding with 0x66 and REX.W

[raaller]

Your checklist for this pull request

• I've documented or updated the documentation of every API function and struct this PR changes.
• I've added tests that prove my fix is effective or that my feature works (if possible)

Detailed description

In 64-bit mode, 66 48 C2/C3 selected the 16-bit near-RET decoder entry. For C2, this also made the decoder expect a four-byte immediate even though RET always encodes its stack adjustment as imm16.

Test plan

Add regression tests for both C2 and C3 with the operand-size override and REX.W:

• 66 48 C2 3B 01 decodes as ret 0x13b with a two-byte immediate and the mode64 group.
• 66 48 C3 decodes as ret with the mode64 group.

All tests passed

Closing issues

Closes #3038.