[ESIMD] Add writeable subsript operator for simd_view

[DenisBakhvalov]

The primary goal for this patch is to allow writing through
simd_view subscript operator, e.g.:

  simd<int, 4> v = 1;
  auto vv = v.select<2, 1>(0);
  vv[0] = 42;

All the other changes are required to make that happen:

1. One more specialization for nested simd_view class with len=1.

2. Added explicit definition of BINOP for simd_view_impl with
   a scalar value because of the following code:

  simd<ushort, 64> s = 0;
  auto g = s.bit_cast_view<ushort, 4, 16>();
  auto x = g.row(1) - (g.row(1))[0];

Here g.row(1) return simd_view and (g.row(1))[0] returns
simd_view of size 1. To use any binops defined in simd_view_impl
before this patch we would need to implicitly convert RHS to
scalar and then convert scalar to vector type. Two implicit
conversions are not allowed according to C++ language rules,
hence new BINOP operator definition for simd_view_impl.

3. I also added additional template parameter for simd_view
   specialization for lenght=1 to support the following code:

  simd<int, 1> s = 0;
  float f = s.bit_cast_view<float>();

This code is OK. The result of bit_cast_view should be mapped
to specialization of simd_view with length 1, so conversion to
scalar is allowed. To enable such mapping, I fixed the definition
of region_base_1. Previously we had zeros for StrideY and StrideX
but in fact, we don't care about their values, they can take any
values.

[DenisBakhvalov]

Corresponding E2E test is here: intel/llvm-test-suite#415