fixup! add comment to test

Author: topperc

llvm/test/CodeGen/RISCV/rvv/pr125306.ll

@@ -1,6 +1,14 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
 ; RUN: llc < %s -mtriple=riscv64 -mattr=+m,+v | FileCheck %s
 
+; Test for an "Invalid size request on a scalable vector". Attempts to reduce
+; the test faurther were not successful. The failure requires a shuffle with 2
+; scalable->fixed extracts from the same vector. 0 is the only valid index for a
+; scalable->fixed extract so the 2 extract must be the same. Shuffles with the
+;same source are aggressively canonicalized to a unary shuffle so it requires
+; the extracts to become identical through other optimizations without the
+; shuffle being canonicalized before it is lowered.
+
 define <2 x i32> @main(ptr %0) {
 ; CHECK-LABEL: main:
 ; CHECK:       # %bb.0: # %entry

llvm/utils/TableGen/DAGISelMatcherOpt.cpp

@@ -135,25 +135,6 @@ static void ContractNodes(std::unique_ptr<Matcher> &MatcherPtr,
       // variants.
     }
 
-  // If we have a CheckType/CheckChildType/Record node followed by a
-  // CheckOpcode, invert the two nodes.  We prefer to do structural checks
-  // before type checks, as this opens opportunities for factoring on targets
-  // like X86 where many operations are valid on multiple types.
-  if ((isa<CheckTypeMatcher>(N) || isa<CheckChildTypeMatcher>(N) ||
-       isa<RecordMatcher>(N)) &&
-      isa<CheckOpcodeMatcher>(N->getNext())) {
-    // Unlink the two nodes from the list.
-    Matcher *CheckType = MatcherPtr.release();
-    Matcher *CheckOpcode = CheckType->takeNext();
-    Matcher *Tail = CheckOpcode->takeNext();
-
-    // Relink them.
-    MatcherPtr.reset(CheckOpcode);
-    CheckOpcode->setNext(CheckType);
-    CheckType->setNext(Tail);
-    return ContractNodes(MatcherPtr, CGP);
-  }
-
   ContractNodes(N->getNextPtr(), CGP);
 
   // If we have a MoveParent followed by a MoveChild, we convert it to