[DAGCombiner] Handle extending EXTRACT_VECTOR_ELTs in calculateByteProvider (#83963)

An EXTRACT_VECTOR_ELT can extend the element to the width of its result
type, leaving the high bits undefined. Previously if we attempted to
query the bytes in these high bits we would recurse and hit an
assertion. This fixes it by bailing if the index is outside of the
vector element size.

I think the assertion Index < ByteWidth may still be incorrect, since
ByteWidth is calculated from Op.getValueSizeInBits(). I believe this
should be Op.getScalarValueSizeInBits() whenever VectorIndex is set
since we're querying the element now, not the vector. But I couldn't
think of a test case to trigger it. It can be addressed in a follow-up
patch.

Fixes #83920

Author: lukel97

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -8627,6 +8627,10 @@ calculateByteProvider(SDValue Op, unsigned Index, unsigned Depth,
     if (NarrowBitWidth % 8 != 0)
       return std::nullopt;
     uint64_t NarrowByteWidth = NarrowBitWidth / 8;
+    // EXTRACT_VECTOR_ELT can extend the element type to the width of the return
+    // type, leaving the high bits undefined.
+    if (Index >= NarrowByteWidth)
+      return std::nullopt;
 
     // Check to see if the position of the element in the vector corresponds
     // with the byte we are trying to provide for. In the case of a vector of

llvm/test/CodeGen/RISCV/rvv/pr83920.ll

@@ -0,0 +1,16 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 4
+; RUN: llc < %s -mtriple=riscv64 -mattr=+v | FileCheck %s
+
+define i8 @or_load_combine(ptr %p) {
+; CHECK-LABEL: or_load_combine:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetivli zero, 2, e8, mf8, ta, ma
+; CHECK-NEXT:    vle8.v v8, (a0)
+; CHECK-NEXT:    vmv.x.s a0, v8
+; CHECK-NEXT:    ori a0, a0, 1
+; CHECK-NEXT:    ret
+  %load = load <2 x i8>, ptr %p
+  %extract = extractelement <2 x i8> %load, i64 0
+  %or = or i8 %extract, 1
+  ret i8 %or
+}