[X86] Cleanup type conversion of 64-bit load-store pairs.

Summary:
Simplify and generalize chain handling and search for 64-bit load-store pairs.
Nontemporal test now converts 64-bit integer load-store into f64 which it realizes directly instead of splitting into two i32 pairs.

Reviewers: craig.topper, spatel

Reviewed By: craig.topper

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D40918

llvm-svn: 320505