[turbofan] Better narrow the derived type for the right shift operation.
Currently the derived type of a right shift does not narrow the input
type based on the actual shift amount - well it does some narrowing
but more can be down. For patterns such as u32[i>>2], which is very
common is asm.js code, this limits the ability to later prove that an
index bounds check is unnecessary which can have a significant
performance impact.
Review URL: https://codereview.chromium.org/
873143002
Cr-Commit-Position: refs/heads/master@{#26270}