[Inliner] Teach inliner to merge 'min-legal-vector-width' function attribute

When we inline a function with a min-legal-vector-width attribute we need to make sure the caller also ends up with at least that vector width.

This patch is necessary to make always_inline functions like intrinsics propagate their min-legal-vector-width. Though nothing uses min-legal-vector-width yet.

A future patch will add heuristics to preventing inlining with different vector width mismatches. But that code would need to be in inline cost analysis which is separate from the code added here.

Differential Revision: https://reviews.llvm.org/D49162

llvm-svn: 337844