RegBankSelect: Allow targets to introduce control flow for mapping

For AMDGPU, if an operand requires an SGPR but is only available as a
VGPR, a loop needs to be introduced to execute the instruction with
each unique combination of values across all lanes. The rest of the
instructions in the block will be moved to a new block following the
loop. Check if the next instruction's parent changed, and update the
iterators and insertion block if this happened.

Tests will be included in a future patch.

llvm-svn: 354591