[SelectionDAG] Remove exit-on-error flag from test (PR27765)

The exit-on-error flag in the ARM test is necessary in order to avoid an
unreachable in the DAGTypeLegalizer, when trying to expand a physical register.
We can also avoid this situation by introducing a bitcast early on, where the
invalid scalar-to-vector conversion is detected.

We also add a test for PowerPC, which goes through a similar code path in the
SelectionDAGBuilder.

Fixes PR27765.

Differential Revision: http://reviews.llvm.org/D21061

llvm-svn: 272644