bpf: fix a bug in trunc-op optimization

Previous implementation for per-function scope
is incorrect and too conservative.

Signed-off-by: Yonghong Song <yhs@fb.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316481 91177308-0d34-0410-b5e6-96231b3b80d8