The function that generated a pre-shifted immediate didn't account for the
instruction with post-shift being unencodable. Fix this by passing
information about the target instruction, and use it to limit the application
of pre-shift.

BUG=chromium:725858

Change-Id: Ia0f70b2ea057975d90162aa6889f15b553acd321
Review-Url: https://codereview.chromium.org/2922173004
Cr-Commit-Position: refs/heads/master@{#45911}