[ConstantFold][SVE] Fix constant folding for bitcast.

Do not iterate on scalable vector type in BitCastConstantVector.
Continuation work of D70985, D71147.

Support for folding bitcast into splat value is kept in D74095, as
it depends on D71637.

Differential Revision: https://reviews.llvm.org/D71389

Author: huihzhang

llvm/lib/IR/ConstantFold.cpp

@@ -47,6 +47,11 @@ static Constant *BitCastConstantVector(Constant *CV, VectorType *DstTy) {
   if (CV->isAllOnesValue()) return Constant::getAllOnesValue(DstTy);
   if (CV->isNullValue()) return Constant::getNullValue(DstTy);
 
+  // Do not iterate on scalable vector. The num of elements is unknown at
+  // compile-time.
+  if (DstTy->isScalable())
+    return nullptr;
+
   // If this cast changes element count then we can't handle it here:
   // doing so requires endianness information.  This should be handled by
   // Analysis/ConstantFolding.cpp

llvm/test/Analysis/ConstantFolding/bitcast.ll

@@ -0,0 +1,12 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -constprop -S -verify | FileCheck %s
+
+define <vscale x 4 x float> @bitcast_scalable_constant() {
+; CHECK-LABEL: @bitcast_scalable_constant(
+; CHECK-NEXT:    ret <vscale x 4 x float> bitcast (<vscale x 4 x i32> shufflevector (<vscale x 4 x i32> insertelement (<vscale x 4 x i32> undef, i32 1, i32 0), <vscale x 4 x i32> undef, <vscale x 4 x i32> zeroinitializer) to <vscale x 4 x float>)
+;
+  %i1 = insertelement <vscale x 4 x i32> undef, i32 1, i32 0
+  %i2 = shufflevector <vscale x 4 x i32> %i1, <vscale x 4 x i32> undef, <vscale x 4 x i32> zeroinitializer
+  %i3 = bitcast <vscale x 4 x i32> %i2 to <vscale x 4 x float>
+  ret <vscale x 4 x float> %i3
+}