bpf: Add x86-64 JIT support for bpf_addr_space_cast instruction.

bpf: Add x86-64 JIT support for bpf_addr_space_cast instruction.

LLVM generates bpf_addr_space_cast instruction while translating
pointers between native (zero) address space and
__attribute__((address_space(N))).
The addr_space=1 is reserved as bpf_arena address space.

rY = addr_space_cast(rX, 0, 1) is processed by the verifier and
converted to normal 32-bit move: wX = wY

rY = addr_space_cast(rX, 1, 0) has to be converted by JIT:

aux_reg = upper_32_bits of arena->user_vm_start
aux_reg <<= 32
wX = wY // clear upper 32 bits of dst register
if (wX) // if not zero add upper bits of user_vm_start
  wX |= aux_reg

JIT can do it more efficiently:

mov dst_reg32, src_reg32  // 32-bit move
shl dst_reg, 32
or dst_reg, user_vm_start
rol dst_reg, 32
xor r11, r11
test dst_reg32, dst_reg32 // check if lower 32-bit are zero
cmove r11, dst_reg	  // if so, set dst_reg to zero
			  // Intel swapped src/dst register encoding in CMOVcc

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20240308010812.89848-5-alexei.starovoitov@gmail.com