[BPF] fix a CO-RE issue with -mattr=+alu32

Ilya Leoshkevich (<iii@linux.ibm.com>) reported an issue that
with -mattr=+alu32 CO-RE has a segfault in BPF MISimplifyPatchable
pass.

The pattern will be transformed by MISimplifyPatchable
pass looks like below:
  r5 = ld_imm64 @"b:0:0$0:0"
  r2 = ldw r5, 0
  ... r2 ... // use r2
The pass will remove the intermediate 'ldw' instruction
and replacing all r2 with r5 likes below:
  r5 = ld_imm64 @"b:0:0$0:0"
  ... r5 ... // use r5
Later, the ld_imm64 insn will be replaced with
  r5 = <patched immediate>
for field relocation purpose.

With -mattr=+alu32, the input code may become
  r5 = ld_imm64 @"b:0:0$0:0"
  w2 = ldw32 r5, 0
  ... w2 ... // use w2
Replacing "w2" with "r5" is incorrect and will
trigger compiler internal errors.

To fix the problem, if the register class of ldw* dest
register is sub_32, we just replace the original ldw*
register with:
  w2 = w5
Directly replacing all uses of w2 with in-place
constructed w5 for the use operand seems not working in all cases.

The latest kernel will have -mattr=+alu32 on by default,
so added this flag to all CORE tests.
Tested with latest kernel bpf-next branch as well with this patch.

Differential Revision: https://reviews.llvm.org/D69438