expr: fix misinterpretation of shifts

expr: fix misinterpretation of shifts

It is fairly clear from the surrounding code that these instances were writing
to the wrong destination and, in some cases, reading from the wrong sources. The
effect of this bug was a little slippery as the underlying storage is a union
with `integer` and `floating` occupying the same storage, so this change does
not alter which bytes are stored to. Additionally users rarely left or right
shift within GVPR scripts, so it is unlikely this affected any real world code.

Gitlab: Fixes #2103