Remove dead TLI arg of isKnownNonNull and propagate deadness. NFC.

This actually uncovered a surprisingly large chain of ultimately unused
TLI args.
From what I can gather, this argument is a remnant of when
isKnownNonNull would look at the TLI directly.
The current approach seems to be that InferFunctionAttrs runs early in
the pipeline and uses TLI to annotate the TLI-dependent non-null
information as return attributes.

This also removes the dependence of functionattrs on TLI altogether.

llvm-svn: 274455