[X86] Add test cases inspired by PR38840.

These are test cases inspired by sequences like below for extracting the same bit from every vector element and checking for all zeros/ones.

define i1 @and256_x8(<8 x i32>) {
    %a = trunc <8 x i32> %0 to <8 x i1>
    %b = bitcast <8 x i1> %a to i8
    %d = icmp eq i8 %b, -1
    ret i1 %d
}

This is what the above looks like after InstCombine.

define i1 @and256_x8_opt(<8 x i32>) {
  %2 = and <8 x i32> %0, <i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1>
  %a = icmp ne <8 x i32> %2, zeroinitializer
  %b = bitcast <8 x i1> %a to i8
  %d = icmp eq i8 %b, -1
  ret i1 %d
}

llvm-svn: 341908