AMDGPU/GlobalISel: Use same builder/observer in post-legalizer-combiner

Change match/apply functions into methods of new target specific combiner
helper class. Use reference to MachineIRBuilder from helper instead of
constructing new MachineIRBuilder each time new instruction needs to made.
Allows correct tracking of newly created instructions.

Differential Revision: https://reviews.llvm.org/D90623