bpf: Tighten the immediate predication for 32-bit alu instructions

These 32-bit ALU insn patterns which takes immediate as one operand were
initially added to enable AsmParser support, and the AsmMatcher uses "ins"
and "outs" fields to deduct the operand constraint.

However, the instruction selector doesn't work the same as AsmMatcher. The
selector will use the "pattern" field for which we are not setting the
predication for immediate operands correctly.

Without this patch, i32 would eventually means all i32 operands are valid,
both imm and gpr, while these patterns should allow imm only.

Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Reviewed-by: Yonghong Song <yhs@fb.com>
llvm-svn: 325980