[RegisterBankInfo] Ignore InstrMappings that create impossible to repair operands

Summary:
This is a follow-up to r303043.  In computeMapping(), we need to disqualify an
InstrMapping if it would be impossible to repair one of the registers in the
instruction to match the mapping.

This change is needed in order to be able to define an instruction
mapping for G_SELECT for the AMDGPU target and will be tested
by test/CodeGen/AMDGPU/GlobalISel/regbankselect-select.mir

Reviewers: ab, qcolombet, t.p.northover, dsanders

Reviewed By: qcolombet

Subscribers: tpr, llvm-commits

Differential Revision: https://reviews.llvm.org/D49735

llvm-svn: 337882