[IPSCCP] Don't propagate return values of functions marked as noinline.

This tries to address what Hal defined (in the post-commit review of
r293727) a long-standing problem with noinline, where we end up
de facto inlining trivial functions e.g.

__attribute__((noinline)) int patatino(void) { return 5; }

because of return value propagation.

llvm-svn: 293799