[turbofan/x64] Load word64 followed by a shift right 32 -> load (and sign-extend if necessary) high 32bit.

We were missing this optimization in a few cases because TruncateInt64ToInt32 was also interfering.
Also removed the equivalent from simplified-lowering.cc, as the arm64 instruction selector has a similar optimization.

R=jarin@chromium.org

Review-Url: https://codereview.chromium.org/2252333002
Cr-Commit-Position: refs/heads/master@{#38711}