[GlobalISel] Ignore %noreg when applying default regbank mapping.

When computing the mapping for non-generic instructions, we skipped
%noreg operands, because we can't always reason about their banks.

Also skip them when applying the mapping.  Otherwise, we could end
up with mappings that we can't apply.

While there, duplicate an assert to distinguish between the two
error conditions.

llvm-svn: 297201