[Codegen][X86] Modernize/regenerate old tests. NFCI.

Summary:
Switch to FileCheck where possible.
Adjust tests so they can be easily regenerated by update scripts.

Reviewers: craig.topper, spatel, RKSimon

Reviewed By: spatel

Subscribers: MatzeB, qcolombet, arphaman, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71211