Clarify when shift input to MUL operator is used

It was unclear whether it referred to the input or output type being
i32_t. It now is specific to the input type being i32_t.

This aligns with the ERROR_IFs in the pseudocode.

Signed-off-by: Eric Kunze <eric.kunze@arm.com>
Change-Id: I348db4af96d4c2b1b2b34c8ba8791cb2e818ab45