bpf: fix a bug in bpf-isel trunc-op optimization

In BPF backend, we try to optimize away redundant
trunc operations so that kernel verifier rewrite
remains valid. Previous implementation only works
for a single function.

This patch fixed the issue for multiple functions.
It clears internal map data structure before
performing optimization for each function.

Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
llvm-svn: 316469