[AMDGPU][GlobalISel] Legalize G_MUL for non-standard types

Legalizing G_MUL for non-standard types (like i33) generated an error. Putting
minScalar and maxScalar instead of clampScalar. Also using new rule, instead
of widening to the next power of 2, widen to the next multiple of the passed
argument (32 in this case), so instead of widening i65 to i128, we widen it to
i96.

Patch by: Mateja Marjanovic

Differential Revision: https://reviews.llvm.org/D109228