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}