[NVPTX] Honor alignment on vector loads/stores

We were not considering the stated alignment on vector loads/stores,
leading us to generate vector instructions even when we do not have
sufficient alignment.

Now, for IR like:

  %1 = load <4 x float>, <4 x float>* %ptr, align 4

we will generate correct, conservative PTX like:

  ld.f32 ... [%ptr]
  ld.f32 ... [%ptr+4]
  ld.f32 ... [%ptr+8]
  ld.f32 ... [%ptr+12]

Or if we have an alignment of 8 (for example), we can
generate code like:

  ld.v2.f32 ... [%ptr]
  ld.v2.f32 ... [%ptr+8]