[GlobalISel] combine trunc(trunc) pattern

Summary:
Legalization can introduce the trunc(trunc) pattern. This can cause
problems if one of these intermediate truncs is not legal.
Combine truncs of this pattern, if the resulting trunc is legal.

Reviewers: arsenm, aemerson, dsanders

Reviewed By: arsenm

Subscribers: jvesely, wdng, nhaehnle, rovka, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76601