bpf: add " ll" in the LD_IMM64 asmstring

This partially revert previous fix in commit f5858045aa0b
("bpf: proper print imm64 expression in inst printer").

In that commit, the original suffix "ll" is removed from
LD_IMM64 asmstring. In the customer print method, the "ll"
suffix is printed if the rhs is an immediate. For example,
"r2 = 5ll" => "r2 = 5ll", and "r3 = varll" => "r3 = var".

This has an issue though for assembler. Since assembler
relies on asmstring to do pattern matching, it will not
be able to distiguish between "mov r2, 5" and
"ld_imm64 r2, 5" since both asmstring is "r2 = 5".
In such cases, the assembler uses 64bit load for all
"r = <val>" asm insts.

This patch adds back " ll" suffix for ld_imm64 with one
additional space for "#reg = #global_var" case.

Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
llvm-svn: 312978