spirv: Implement vload[a]_half[n] and vstore[a]_half[n][_r]

Note, the aligned versions aren't handled specially yet.

The float16buffer capability is now at least partially supported after
this patch, so move it to be supported when kernels are supported.

v2 (Jason Ekstrand):
 - A few cosmetic cleanups around type/base_type
 - Rebased on top of the big SPIR-V SSA value rework
 - Use the new version of the conversion helpers

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6945>
4 files changed