-Wtautological-bitwise-compare error in arm-tdep.c

[Simon Marchi]

We get this error in arm-tdep.c when building with clang 11:

  CXX    arm-tdep.o
/home/smarchi/src/binutils-gdb/gdb/arm-tdep.c:934:29: error: bitwise comparison always evaluates to false [-Werror,-Wtautological-bitwise-compare]
          else if ((insn & 0xffb0) == 0xe950    /* ldrd Rt, Rt2,
                   ~~~~~~~~~~~~~~~~^~~~~~~~~

Indeed, this can never evaluate to true.  I tried to look at what the condition
should be, but it's a bit too deep into ARM instruction encoding for me.  I am
pretty sure that the corresponding section in the ARM Architecture Reference
Manual [1] is F3.1.15, if somebody wants to take a stab at it.

Simon

[1] https://static.docs.arm.com/ddi0487/fb/DDI0487F_b_armv8_arm.pdf