Incorrect warning for AVX512 gathers

[Steve Vormwald]

On the current tip of binutils-2_26-branch, gas is emitting an incorrect 
warning 'Warning: index and destination registers should be distinct' when the 
two zmm registers are separated by 16.  For exmaple, zmm19 and zmm3 in 
'vgatherqpd      (%rax,%zmm19,8), %zmm3 {%k1}'.  This warning was added in 
commit 8444f82a1d163171deccfcf014cc31adb81f703b.  The problem appears to be 
that the register_number() function in gas/config/tc-i386.c doesn't check for 
RegVRex flag like it does for the RegRex flag, leading to aliasing issues 
between the low and high 16 registers.

Steven Vormwald