[MLIR][linalg] Make integer matmul ops cast before multiplying

Right now they multiply before casting which means they would frequently
overflow. There are various reasonable ways to do this, but until we
have robust op description infra, this is a simple and safe default. More
careful treatments are likely to be hardware specific, as well (e.g.
using an i8*i8->i16 mul instruction).

Reviewed By: nicolasvasilache, mravishankar

Differential Revision: https://reviews.llvm.org/D97505