x86: Support VEX base opcode length > 1

Intel AMX instructions with 8-bit immediate opcode extension without
operands:

tilerelease, 0, 0x49c0, None, 2, CpuAMX_TILE|Cpu64, Vex|VexOpcode=1|No_bSuf|No_wSuf|No_lSuf|No_sSuf|No_qSuf|No_ldSuf, { 0 }

Update build_vex_prefix to support VEX base opcode length > 1.

	* tc-i386.c (build_vex_prefix): Support VEX base opcode length > 1.
	(md_assemble): Don't process ImmExt without operands.