[IR] Inline some Function accessors

[IR] Inline some Function accessors

I checked that all of these out-of-line methods previously compiled to
simple loads and bittests, so they are pretty good candidates for
inlining. In particular, arg_size() and arg_empty() are popular and are
just two loads, so they seem worth inlining.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@297963 91177308-0d34-0410-b5e6-96231b3b80d8