[X86 TTI] Implement LSV hook

[X86 TTI] Implement LSV hook

Summary:
LSV wants to know the maximum size that can be loaded to a vector register.
On X86, this always matches the maximum register width. Implement this
accordingly and add a test to make sure that LSV can vectorize up to the
maximum permissible width on X86.

Reviewers: delena, arsenm

Reviewed By: arsenm

Subscribers: wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D31504

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@299589 91177308-0d34-0410-b5e6-96231b3b80d8