[X86] Add 64-bit int to float/double conversion with AVX to X86FastISel::X86SelectSIToFP

Summary:
[X86] Teach fast isel to handle i64 sitofp with AVX.

For some reason we only handled i32 sitofp with AVX. But with SSE only we support i64 so we should do the same with AVX.

Also add i686 command lines for the 32-bit tests. 64-bit tests are in a separate file to avoid a fast-isel abort failure in 32-bit mode.

Reviewers: RKSimon, zvi

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39450

llvm-svn: 317102