GlobalISel: Pass MachineIRBuilder to applyMappingImpl

The target should not have to construct MachineIRBuilders during
RegBankSelect (we should perhaps hide the constructors for it). The
pass should own the builder setup with the desired CSE configuration
(although currently the pass does not use the CSE builder, which is
what I want to fix).

https://reviews.llvm.org/D156479