[InstSimplify] fold or+shifted -1 to -1

These are similar to the rotate pattern added with:
dcf659e8
...but we don't have guard ops on the shift amount,
so we don't canonicalize to the intrinsic.

  declare void @llvm.assume(i1)

  define i32 @src(i32 %shamt, i32 %bitwidth) {
    ; subtract must be in range of bitwidth
    %lt = icmp ule i32 %bitwidth, 32
    call void @llvm.assume(i1 %lt)

    %r = lshr i32 -1, %shamt
    %s = sub i32 %bitwidth, %shamt
    %l = shl i32 -1, %s
    %o = or i32 %r, %l
    ret i32 %o
  }

  define i32 @tgt(i32 %shamt, i32 %bitwidth) {
    ret i32 -1
  }

https://alive2.llvm.org/ce/z/aF7WHx