selftests/bpf: check if verifier tracks constants spilled by BPF_ST_MEM

Check that verifier tracks the value of 'imm' spilled to stack by
BPF_ST_MEM instruction. Cover the following cases:
- write of non-zero constant to stack;
- write of a zero constant to stack.
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20230214232030.1502829-3-eddyz87@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/verifier/bpf_st_mem.c

+{
+	"BPF_ST_MEM stack imm non-zero",
+	.insns = {
+	BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 42),
+	BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_10, -8),
+	BPF_ALU64_IMM(BPF_ADD, BPF_REG_0, -42),
+	/* if value is tracked correctly R0 is zero */
+	BPF_EXIT_INSN(),
+	},
+	.result = ACCEPT,
+	/* Use prog type that requires return value in range [0, 1] */
+	.prog_type = BPF_PROG_TYPE_SK_LOOKUP,
+	.expected_attach_type = BPF_SK_LOOKUP,
+	.runs = -1,
+},
+{
+	"BPF_ST_MEM stack imm zero",
+	.insns = {
+	/* mark stack 0000 0000 */
+	BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
+	/* read and sum a few bytes */
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_LDX_MEM(BPF_B, BPF_REG_1, BPF_REG_10, -8),
+	BPF_ALU64_REG(BPF_ADD, BPF_REG_0, BPF_REG_1),
+	BPF_LDX_MEM(BPF_B, BPF_REG_1, BPF_REG_10, -4),
+	BPF_ALU64_REG(BPF_ADD, BPF_REG_0, BPF_REG_1),
+	BPF_LDX_MEM(BPF_B, BPF_REG_1, BPF_REG_10, -1),
+	BPF_ALU64_REG(BPF_ADD, BPF_REG_0, BPF_REG_1),
+	/* if value is tracked correctly R0 is zero */
+	BPF_EXIT_INSN(),
+	},
+	.result = ACCEPT,
+	/* Use prog type that requires return value in range [0, 1] */
+	.prog_type = BPF_PROG_TYPE_SK_LOOKUP,
+	.expected_attach_type = BPF_SK_LOOKUP,
+	.runs = -1,
+},