[AMDGPU] Remove exit-on-error in test (PR27761)

The exit-on-error flag was necessary in order to avoid an assertion when
handling DYNAMIC_STACKALLOC nodes in SelectionDAGLegalize.

We can avoid the assertion by creating some dummy nodes. This enables us to
remove the exit-on-error flag on the first 2 run lines (SI), but on the third
run line (R600) we would run into another assertion when trying to reserve
indirect registers. This patch also replaces that assertion with an early exit
from the function.

Fixes PR27761.

Differential Revision: http://reviews.llvm.org/D20852

llvm-svn: 273550