[GlobalISel] Fix known bits for G_ASSERT_ALIGN.

I don't know what was going on originally with these tests. It seems reasonable
to have the immediate be the same byte alignment unit as the IR, in which case
we need to take the log2 in order to set the right number of low bits.

This fixes a miscompile in chromium.

Differential Revision: https://reviews.llvm.org/D134380