[X86] Generate testl instruction through truncates.

Summary:
This was introduced in D42646 but ended up being reverted because the original implementation was buggy.

Depends on D42646

Reviewers: craig.topper, niravd, spatel, hfinkel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D42741

llvm-svn: 323899