x86-64 allows R12 as an index register in SIB addressing.

As R12 is wired to the current thread ptr, this fix has little practical
impact, but corrects the inconsistency that "lea Rn,[rax+r12]" is
effectively identical to "lea Rn,[r12+rax]" except the former caused
the assembler to crash and the disassembler to be confused.