[InlineAsm] Update the min-legal-vector-width function attribute based on...

[InlineAsm] Update the min-legal-vector-width function attribute based on inputs and outputs to inline assembly

Summary:
Another piece of my ongoing to work for prefer-vector-width.

min-legal-vector-width will eventually be used by the X86 backend to know whether it needs to make 512 bits type legal when prefer-vector-width=256. If the user used inline assembly that passed in/out a 512-bit register, we need to make sure 512 bits are considered legal. Otherwise we'll get an assert failure when we try to wire up the inline assembly to the rest of the code.

This patch just checks the LLVM IR types to see if they are vectors and then updates the attribute based on their total width. I'm not sure if this is the best way to do this or if there's any subtlety I might have missed. So if anyone has other opinions on how to do this I'm open to suggestions.

Reviewers: chandlerc, rsmith, rnk

Reviewed By: rnk

Subscribers: eraman, cfe-commits

Differential Revision: https://reviews.llvm.org/D50678

llvm-svn: 339721