Register R12 requires special encoding in modrm bytes - Avoid using it to simplify x64 code gen

[Alan-Jowett]

Register R12 requires special encoding in modrm byte - Avoid using it to simplify x64 code gen.

0: 45 8b 3b mov r15d,DWORD PTR [r11]
0: 45 8b 3c 24 mov r15d,DWORD PTR [r12]

For reason not yet understood, a move operations with R12 as indirect source results in a 4 byte instruction.

Switch the register mapping to avoid R12.

[Alan-Jowett]

More background on this:
R12 and RSP use the same register encoding (excluding the bit in REX).
R13 and RBP use the same register encoding (excluding the bit in REX).

emit_modrm_and_displacement correctly handles encoding R13 as an EBP relative addressing, but doesn't do RSP relative addressing correctly. This results in malformed instruction.

ubpf/vm/ubpf_jit_x86_64.h

Line 125 in 221bbe0

if (d == 0 && (m & 7) != RBP) {