[DAG] enhance computeKnownBits to handle SHL with vector splat constant

Also, use APInt to avoid crashing on types larger than vNi64.

llvm-svn: 284874
diff --git a/llvm/test/CodeGen/X86/combine-shl.ll b/llvm/test/CodeGen/X86/combine-shl.ll
index 3f3871e..64ed1ce 100644
--- a/llvm/test/CodeGen/X86/combine-shl.ll
+++ b/llvm/test/CodeGen/X86/combine-shl.ll
@@ -61,16 +61,12 @@
 define <4 x i32> @combine_vec_shl_known_zero0(<4 x i32> %x) {
 ; SSE-LABEL: combine_vec_shl_known_zero0:
 ; SSE:       # BB#0:
-; SSE-NEXT:    pxor %xmm1, %xmm1
-; SSE-NEXT:    pblendw {{.*#+}} xmm0 = xmm1[0],xmm0[1],xmm1[2],xmm0[3],xmm1[4],xmm0[5],xmm1[6],xmm0[7]
-; SSE-NEXT:    pslld $16, %xmm0
+; SSE-NEXT:    xorps %xmm0, %xmm0
 ; SSE-NEXT:    retq
 ;
 ; AVX-LABEL: combine_vec_shl_known_zero0:
 ; AVX:       # BB#0:
-; AVX-NEXT:    vpxor %xmm1, %xmm1, %xmm1
-; AVX-NEXT:    vpblendw {{.*#+}} xmm0 = xmm1[0],xmm0[1],xmm1[2],xmm0[3],xmm1[4],xmm0[5],xmm1[6],xmm0[7]
-; AVX-NEXT:    vpslld $16, %xmm0, %xmm0
+; AVX-NEXT:    vxorps %xmm0, %xmm0, %xmm0
 ; AVX-NEXT:    retq
   %1 = and <4 x i32> %x, <i32 4294901760, i32 4294901760, i32 4294901760, i32 4294901760>
   %2 = shl <4 x i32> %1, <i32 16, i32 16, i32 16, i32 16>