ac/nir: Use context-specific LLVM types

LLVMInt*Type() return types from the global context and therefore are
not safe for use in other contexts. Use types from our own context
instead.

Fixes frequent crashes seen when doing multithreaded pipeline creation.

Fixes: 4d0b02bb "ac: add support for 16bit load_push_constant"
Fixes: 7e7ee826 "ac: add support for 16bit buffer loads"
Cc: "18.2" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Alex Smith <asmith@feralinteractive.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit ca83d51c)