BPF: Implement TTI.IntImmCost() properly

This patch implemented TTI.IntImmCost() properly.
Each BPF insn has 32bit immediate space, so for any immediate
which can be represented as 32bit signed int, the cost
is technically free. If an int cannot be presented as
a 32bit signed int, a ld_imm64 instruction is needed
and a TCC_Basic is returned.

This change is motivated when we observed that
several bpf selftests failed with latest llvm trunk, e.g.,
  #10/16 strobemeta.o:FAIL
  #10/17 strobemeta_nounroll1.o:FAIL
  #10/18 strobemeta_nounroll2.o:FAIL
  #10/19 strobemeta_subprogs.o:FAIL
  #96 snprintf_btf:FAIL

The reason of the failure is due to that
SpeculateAroundPHIsPass did aggressive transformation
which alters control flow for which currently verifer
cannot handle well. In llvm12, SpeculateAroundPHIsPass
is not called.

SpeculateAroundPHIsPass relied on TTI.getIntImmCost()
and TTI.getIntImmCostInst() for profitability
analysis. This patch implemented TTI.getIntImmCost()
properly for BPF backend which also prevented
transformation which caused the above test failures.

Differential Revision: https://reviews.llvm.org/D96448