Fix switch-lower-peel-top-case.ll isel pass is not registered error

The test was doing -stop-after=isel, but that pass is actually the
AMDGPUDAGToDAGISel pass, which might not be built when targeting x86_64.
This changes the test to -stop-after=expand-isel-pseudos instead.

Follow-up to r318202.

llvm-svn: 318220