[test] Pin backedge-id-bug-xfail.ll to legacy PM

The new PM doesn't have region passes, so this doesn't really make sense in a NPM context.

Author: aeubanks

llvm/test/Transforms/StructurizeCFG/AMDGPU/backedge-id-bug-xfail.ll

@@ -1,5 +1,5 @@
 ; XFAIL: *
-; RUN: opt -mtriple=amdgcn-amd-amdhsa -S -structurizecfg -verify-region-info %s
+; RUN: opt -mtriple=amdgcn-amd-amdhsa -S -structurizecfg -verify-region-info %s -enable-new-pm=0
 
 ; FIXME: Merge into backedge-id-bug
 ; Variant which has an issue with region construction