mips: Fix "XPASS" test results by removing 'not' commands

These tests are asserting and crashing for me, and 'not' sees that as a
non-zero exit code instead of a signal code for obscure Windows reasons.
This causes the test to pass, giving me an unclean 'ninja check'.

The test is already XFAILd, so just run the test without 'not' and let
lit handle the failure.

llvm-svn: 226958