GlobalISel: Help reduce operation width for instruction with two results.

The function `reduceOperationWidth` helps to legalize a vector
operation either by narrowing its type or by scalarizing the
operation itself. It currently supports instructions with one result.
This patch, in addition allows the same for instructions with two
results (for instance, G_SDIVREM).

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D100725