Currently this code exists in widenScalar for G_MERGE_VALUE
sources. I'm not sure if the existing expansion in widenScalar should
be removed or not. The widenScalar variant tries to extend to the
requested size, but this just uses the original bitwidth.
I tried to use some of the new tablegen features to avoid creating
different operand list permutations, but I still don't see a way to
programmatically build a source pattern dag.
Also add GlobalISel tests, which now all import successfully.
Some of the fneg fold tests are incorrect, which need to be fixed in a
future commit
Mircea Trofin