[BPF] support for BPF_ST instruction in codegen

Generate store immediate instruction when CPUv4 is enabled.
For example:

    $ cat test.c
    struct foo {
      unsigned char  b;
      unsigned short h;
      unsigned int   w;
      unsigned long  d;
    };
    void bar(volatile struct foo *p) {
      p->b = 1;
      p->h = 2;
      p->w = 3;
      p->d = 4;
    }

    $ clang -O2 --target=bpf -mcpu=v4 test.c -c -o - | llvm-objdump -d -
    ...
    0000000000000000 <bar>:
           0:	72 01 00 00 01 00 00 00	*(u8 *)(r1 + 0x0) = 0x1
           1:	6a 01 02 00 02 00 00 00	*(u16 *)(r1 + 0x2) = 0x2
           2:	62 01 04 00 03 00 00 00	*(u32 *)(r1 + 0x4) = 0x3
           3:	7a 01 08 00 04 00 00 00	*(u64 *)(r1 + 0x8) = 0x4
           4:	95 00 00 00 00 00 00 00	exit

Take special care to:
- apply `BPFMISimplifyPatchable::checkADDrr` rewrite for BPF_ST
- validate immediate value when BPF_ST write is 64-bit:
  BPF interprets `(BPF_ST | BPF_MEM | BPF_DW)` writes as writes with
  sign extension. Thus it is fine to generate such write when
  immediate is -1, but it is incorrect to generate such write when
  immediate is +0xffff_ffff.

Differential Revision: https://reviews.llvm.org/D140804