Revert "[X86][DAGISel] Don't widen shuffle element with AVX512"

This reverts commit 5fb41342.

This patch is causing crashes when building llvm-test-suite when
optimizing for CPUs with AVX512.

Reproducer crashing with llc:

    target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
    target triple = "x86_64-apple-macosx"

    define i32 @test(<32 x i32> %0) #0 {
    entry:
      %1 = mul <32 x i32> %0, <i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1>
      %2 = tail call i32 @llvm.vector.reduce.add.v32i32(<32 x i32> %1)
      ret i32 %2
    }

    ; Function Attrs: nocallback nofree nosync nounwind readnone willreturn
    declare i32 @llvm.vector.reduce.add.v32i32(<32 x i32>) #1

    attributes #0 = { "min-legal-vector-width"="0" "target-cpu"="skylake-avx512" }
    attributes #1 = { nocallback nofree nosync nounwind readnone willreturn }