[GlobalISel] Bail on G_PHI narrowing of odd types (PR48188)

The current narrowing code for G_PHI can only handle the case
where the size is a multiple of the narrow size. If this is not
the case, fall back to SDAG instead of asserting.

Original patch by shepmaster.

Differential Revision: https://reviews.llvm.org/D92446

(cherry picked from commit c35761db)