[naga] Vectorize [un]pack4x{I, U}8[Clamp] on spv

Emits vectorized SPIR-V code for the WGSL functions `unpack4xI8`,
`unpack4xU8`, `pack4xI8`, `pack4xU8`, `pack4xI8Clamp`, and
`pack4xU8Clamp` if `Capability::Int8` is available.

Exploits the following facts about SPIR-V ops:
- `SClamp`, `UClamp`, and `OpUConvert` accept vector arguments, in which
  case results are computed per component; and
- `OpBitcast` can cast between vectors and scalars, with a well-defined
  bit order that matches that required by the WGSL spec, see below.

WGSL spec for `pack4xI8` [1]:

> Component e[i] of the input is mapped to bits 8 x i through 8 x i + 7
> of the result.

SPIR-V spec for `OpBitcast` [2]:

> Within this mapping, any single component of `S` [remark: the type
> with fewer but wider components] (mapping to multiple components of
> `L` [remark: the type with more but narrower components]) maps its
> lower-ordered bits to the lower-numbered components of `L`.

[1] https://www.w3.org/TR/WGSL/#pack4xI8-builtin
[2] https://registry.khronos.org/SPIR-V/specs/unified1/SPIRV.html#OpBitcast

Author: robamler