[RISCV] Fix assertion in lowerEXTRACT_SUBVECTOR

This fixes a crash when lowering an extract_subvector like:

t0:v1i64 = extract_subvector t1:v2i64, 1

Whilst we never need a vslidedown with M1 on scalable vector types, we might
need to do it for v1i64/v1f64, since the smallest container type for it is
nxv1i64/nxv1f64.

The lowering code is still correct for this case, but the assertion was too
strict. The actual invariant we're relying on is that ContainerSubVecVT's LMUL
<= M1, not < M1. Hence why we handled v2i32 fine, because its container type
was nxv1i32 and MF2.

Author: lukel97

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -9804,10 +9804,11 @@ SDValue RISCVTargetLowering::lowerEXTRACT_SUBVECTOR(SDValue Op,
     return Op;
   }
 
-  // Else SubVecVT is a fractional LMUL and may need to be slid down: if
-  // SubVecVT was > M1 then the index would need to be a multiple of VLMAX, and
-  // so would divide exactly.
-  assert(RISCVVType::decodeVLMUL(getLMUL(ContainerSubVecVT)).second);
+  // Else SubVecVT is M1 or smaller and may need to be slid down: if SubVecVT
+  // was > M1 then the index would need to be a multiple of VLMAX, and so would
+  // divide exactly.
+  assert(RISCVVType::decodeVLMUL(getLMUL(ContainerSubVecVT)).second ||
+         getLMUL(ContainerSubVecVT) == RISCVII::VLMUL::LMUL_1);
 
   // If the vector type is an LMUL-group type, extract a subvector equal to the
   // nearest full vector register type.

llvm/test/CodeGen/RISCV/rvv/fixed-vectors-extract-subvector.ll

@@ -857,6 +857,16 @@ define void @extract_v8i1_nxv32i1_16(<vscale x 32 x i1> %x, ptr %y) {
   ret void
 }
 
+define <1 x i64> @extract_v1i64_v2i64_1(<2 x i64> %x) {
+; CHECK-LABEL: extract_v1i64_v2i64_1:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    vsetivli zero, 1, e64, m1, ta, ma
+; CHECK-NEXT:    vslidedown.vi v8, v8, 1
+; CHECK-NEXT:    ret
+  %v = call <1 x i64> @llvm.vector.extract.v1i64.v2i64(<2 x i64> %x, i64 1)
+  ret <1 x i64> %v
+}
+
 declare <2 x i1> @llvm.vector.extract.v2i1.v64i1(<64 x i1> %vec, i64 %idx)
 declare <8 x i1> @llvm.vector.extract.v8i1.v64i1(<64 x i1> %vec, i64 %idx)