aco: check alignment of non-subdword registers in get_reg_specified()

When splitting a v6b vector into v1 and v2b components, we should ensure
the v1 definition doesn't start at the upper half.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4772>

src/amd/compiler/aco_register_allocation.cpp

@@ -877,6 +877,8 @@ bool get_reg_specified(ra_ctx& ctx,
 {
    if (rc.is_subdword() && reg.byte() && !instr_can_access_subdword(instr))
       return false;
+   if (!rc.is_subdword() && reg.byte())
+      return false;
 
    uint32_t size = rc.size();
    uint32_t stride = 1;