[BPF] disable ReduceLoadWidth during SelectionDag phase

The compiler may transform the following code
  ctx = ctx + reloc_offset
  ... (*(u32 *)ctx) & 0x8000 ...
to
  ctx = ctx + reloc_offset
  ... (*(u8 *)(ctx + 1)) & 0x80 ...
where reloc_offset will be replaced with a constant during
AsmPrinter phase.

The above transformed code will be rejected the kernel verifier
as it does not allow
  *(type *)((ctx + non_zero_offset1) + non_zero_offset2)
style access pattern.

It is hard at SelectionDag phase to identify whether a load
is related to context or not. Sometime, interprocedure analysis
may be needed. So let us simply prevent such optimization
from happening.

Differential Revision: https://reviews.llvm.org/D73997