x86: bpf_jit_comp: Reduce is_ereg() code size

x86: bpf_jit_comp: Reduce is_ereg() code size

Use the (1 << reg) & mask trick to reduce code size.

x86-64 size difference -O2 without profiling for various
gcc versions:

$ size arch/x86/net/bpf_jit_comp.o*
   text    data     bss     dec     hex filename
   9266       4       0    9270    2436 arch/x86/net/bpf_jit_comp.o.4.4.new
  10042       4       0   10046    273e arch/x86/net/bpf_jit_comp.o.4.4.old
   9109       4       0    9113    2399 arch/x86/net/bpf_jit_comp.o.4.6.new
   9717       4       0    9721    25f9 arch/x86/net/bpf_jit_comp.o.4.6.old
   8789       4       0    8793    2259 arch/x86/net/bpf_jit_comp.o.4.7.new
  10245       4       0   10249    2809 arch/x86/net/bpf_jit_comp.o.4.7.old
   9671       4       0    9675    25cb arch/x86/net/bpf_jit_comp.o.4.9.new
  10679       4       0   10683    29bb arch/x86/net/bpf_jit_comp.o.4.9.old

Signed-off-by: Joe Perches <joe@perches.com>
Tested-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>