bpf: New disassembler testcases for 32-bit subregister support

This patch test disassembler output for load/store instructions when
-mattr=+alu32 specified for which we want to use "w" register format.

Also, this patch extended the existing insn-unit.s and insn-unit-32.s to
make sure disassemblers for all other instructions are not affected.

Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Reviewed-by: Yonghong Song <yhs@fb.com>
llvm-svn: 325993